Patent Publication Number: US-2018051228-A1

Title: Lubricating oil composition for four stroke engine

Description:
TECHNICAL FIELD 
     The present invention relates to a lubricating oil composition for four-stroke cycle engine, and for example, it relates to a lubricating oil composition which is suitably usable for a four-stroke cycle engine to be continuously operated under a high load in shipping or fixed-type power generation equipment or the like. 
     BACKGROUND ART 
     As for engine oils to be used for a gas cogeneration system or power generation equipment, in order to conform to a high load, those having a high-viscosity grade such as SAE J300 #40 are needed, and there is a case where even if a neutral oil (for example, a 500N mineral oil) is used as the base oil, a sufficient viscosity is not obtained. For that reason, conventionally, a high-viscosity base material, such as a bright stock, an ethylene/propylene copolymer (OCP), etc., is blended therein in some cases. 
     In addition, conventionally, for example, as disclosed in PTL 1, with respect to lubricating oil compositions to be used for diesel engines for shipping or engines for power generation, there are known those in which by blending a predetermined thickener in a Fischer-Tropsch derived (FT derived) base oil having a kinematic viscosity at 100° C. of 6 to 10 mm 2 /s, the lubricant-fuel compatibility is improved. Here, as the thickener, one selected from a FT derived base oil having a kinematic viscosity at 100° C. in the range of 15 to 30 mm 2 /s, a bright stock, a deasphalted cylinder oil, polyisobutylene, and a mixture thereof is used. 
     Furthermore, PTL 2 discloses that in a lubricating oil composition to be used for internal combustion engines, such as a gasoline engine, a diesel engine, etc., or the like, by blending 0.001 to 1% by weight of a polybutene and/or polyisobutene having a weight average molecular weight of 500,000 to 10,000,000 in a lubricant base oil, the engine detergency is improved. 
     In four-stroke cycle engines, in recent years, for the purpose of improving the engine efficiency, a compression ratio and a turbo boost pressure are increased, and following this, there is a tendency that a combustion temperature or pressure of the engine also becomes high. When the engine is operated at a high temperature under a high pressure, the lubricating oil is degraded, and stains are liable to be generated in an underside, a ring groove, a land portion, or the like of a piston. In consequence, in the lubricating oil for four-stroke engine, high piston detergency tends to be required. Such a requirement is becoming strong, since the operation is performed at a high load factor for a long time, for example, in shipping or fixed-type power generation equipment, and furthermore, a cogeneration system. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 2012-511608 A 
     PTL 2: JP 2007-246771 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional lubricating oils to be used for a gas cogeneration system or an engine for power generation equipment, when blended with the aforementioned high-viscosity base material, such as a bright stock, OCP, etc., or thickener, such a material may generate a sludge at a high temperature. The generation of sludge leads to deterioration of the piston detergency. Accordingly, a high heat-resistant detergent or dispersant is blended, or optimization of an antioxidant is performed in some cases; however, there is a limit in suppressing the generation of sludge or enhancing the piston detergency by using such an additive. 
     In addition, if the polybutene and/or polyisobutene having a high molecular weight is blended in a proportion of 1% by mass or less as disclosed in PTL 2, the generation of sludge cannot be thoroughly prevented from occurring. In addition, the effect of improving the detergency is also limitative. 
     Moreover, for example, if the amount of a metal-based detergent or the like is increased in order to improve the piston detergency, the metal component adheres to engine parts, resulting in a cause of knocking. In consequence, in the gas cogeneration system or engine for power generation equipment, at the time of replacing an oil for example, cleaning should be performed through a manual work in order to remove the metal component adhered to the engine parts or stains of the piston. 
     Furthermore, in recent years, in order to improve the engine efficiency, the turbo boost pressure is increasing, and in a turbocharger, there is a concern that a trouble, such as breakage of the turbocharger to be caused due to formation of a deposit (coking) by degradation of the lubricating oil, etc., is caused. In consequence, in a lubricating oil to be used for an engine equipped with a turbocharger, it is required to suppress the turbo coking. In addition thereto, in engine oils for gas cogeneration system or power generation equipment, there is a case where long draining properties are required. 
     In view of the foregoing circumstances, the present invention has been made, and a problem of the present invention is to provide a lubricating oil composition for four-stroke cycle engine in which even when a base material for thickening is blended, not only the generation of sludge under a high-temperature operation is suppressed, but also various performances can be made satisfactory. 
     Specifically, a problem of the present invention in a first aspect is to provide a lubricating oil composition for four-stroke cycle engine in which even when a base material for thickening is blended, not only the generation of sludge under a high-temperature operation is suppressed, but also the piston detergency is improved, and even when a cleaning work is not frequently performed, it is possible to prevent a trouble, such as knocking, etc., from occurring. 
     Furthermore, a problem of the present invention in a second aspect is to provide a lubricating oil composition for four-stroke cycle engine in which even when a base material for thickening is blended, not only the generation of sludge under a high-temperature operation is suppressed, but also the resistance to turbo coking is enhanced, and even when used in a severe environment, long draining properties are revealed. 
     Solution to Problem 
     In order to solve the aforementioned problems, the present inventors made extensive and intensive investigations. As a result, it has been found that the aforementioned problems can be solved by regulating a sulfated ash content in a lubricating oil composition to a predetermined amount while blending a predetermined amount of a polyolefin having a specified molecular weight and including an isobutene-derived constituting unit, leading to accomplishment of the present invention. The present invention provides the following lubricating oil composition for four-stroke cycle engine. 
     A lubricating oil composition for four-stroke cycle engine, including a base oil (A) and from 0.5 to 10% by mass of a polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000, with a sulfated ash content being from 0.3 to 1.2% by mass. 
     In addition, the present invention also provides a lubricating method of performing lubrication between respective parts in a four-stroke cycle engine with the aforementioned lubricating oil composition for four-stroke cycle engine. 
     Furthermore, the present invention also provides the following production method of a lubricating oil composition for four-stroke cycle engine. 
     A production method of a lubricating oil composition for four-stroke cycle engine, including: blending a base oil (A) with from 0.5 to 10% by mass of a polyolefin (B) based on the total amount of the lubricating oil composition, the polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000; and regulating a sulfated ash content to from 0.3 to 1.2% by mass. 
     In addition, in order to solve the aforementioned problem of the present invention in the first aspect, the present inventors made extensive and intensive investigations. As a result, it has been found that the foregoing problem can be solved by blending a predetermined amount of a polyolefin having a specified molecular weight and including an isobutene-derived constituting unit and also decreasing a sulfated ash content in the lubricating oil composition, leading to accomplishment of the present invention in the following first aspect. The present invention in the first aspect provides the following lubricating oil composition for four-stroke cycle engine. 
     A lubricating oil composition for four-stroke cycle engine, including a base oil (A) and from 0.5 to 10% by mass of a polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000, with a sulfated ash content being from 0.3 to 0.7% by mass. 
     In addition, the present invention in the first aspect also provides a lubricating method of performing lubrication between respective parts in a four-stroke cycle engine with the aforementioned lubricating oil composition for four-stroke cycle engine. 
     Furthermore, the present invention in the first aspect also provides the following production method of a lubricating oil composition for four-stroke cycle engine. 
     A production method of a lubricating oil composition for four-stroke cycle engine, including: blending a base oil (A) with from 0.5 to 10% by mass of a polyolefin (B) based on the total amount of the lubricating oil composition, the polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000; and regulating a sulfated ash content to from 0.3 to 0.7% by mass. 
     In order to solve the aforementioned problem of the present invention in the second aspect, the present inventors made extensive and intensive investigations. As a result, it has been found that the foregoing problem can be solved by blending a predetermined amount of a polyolefin having a specified molecular weight and including an isobutene-derived constituting unit and also regulating a sulfated ash content in the lubricating oil composition to a predetermined amount, leading to accomplishment of the present invention in the second aspect. The present invention in the second aspect provides the following lubricating oil composition for four-stroke cycle engine. 
     A lubricating oil composition for four-stroke cycle engine, including a base oil (A) and from 0.5 to 10% by mass of a polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000, with a sulfated ash content being more than 0.7% by mass and 1.2% by mass or less. 
     In addition, the present invention in the second aspect also provides a lubricating method of performing lubrication between respective parts in a four-stroke cycle engine with the aforementioned lubricating oil composition for four-stroke cycle engine. 
     Furthermore, the present invention in the second aspect also provides the following production method of a lubricating oil composition for four-stroke cycle engine. 
     A production method of a lubricating oil composition for four-stroke cycle engine, including: blending a base oil (A) with from 0.5 to 10% by mass of a polyolefin (B) based on the total amount of the lubricating oil composition, the polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000; and regulating a sulfated ash content to more than 0.7% by mass and 1.2% by mass or less. 
     Advantageous Effects of Invention 
     In the present invention, even when a base material for thickening is blended, not only the generation of sludge under a high-temperature operation is suppressed, but also various performances can be made satisfactory. 
     More specifically, in the first aspect, it is possible to provide a lubricating oil composition for four-stroke cycle engine in which even when a base material for thickening is blended, not only the generation of sludge under a high-temperature operation is suppressed, but also the piston detergency is improved, and even when a cleaning work is not frequently performed, it is possible to prevent a trouble, such as knocking, etc., from occurring. 
     Furthermore, in the second aspect, it is possible to provide a lubricating oil composition for four-stroke cycle engine in which even when a base material for thickening is blended, not only the generation of sludge under a high-temperature operation is suppressed, but also the resistance to turbo coking is enhanced, and even when used in a severe environment, long draining properties are revealed. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     A lubricating oil composition for four-stroke cycle engine according to the embodiments of the present invention is one including a base oil (A) and 0.5 to 10% by mass of a polyolefin (B) having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000, with a sulfated ash content being 0.3 to 1.2% by mass. 
     The embodiments of the present invention are hereunder described in more detail. In the following description, an embodiment corresponding to the aforementioned first aspect is described as a first embodiment, and an embodiment corresponding to the aforementioned second aspect is described as a second embodiment. 
     The lubricating oil composition for four-stroke cycle engine of the first embodiment includes at least a base oil (A) and a polyolefin (B), with a sulfated ash content being 0.3 to 0.7% by mass. 
     In addition, the lubricating oil composition for four-stroke cycle engine of the second embodiment includes at least a base oil (A) and a polyolefin (B), with a sulfated ash content being more than 0.7% by mass and 1.2% by mass or less. 
     Hereinafter, the lubricating oil compositions for four-stroke cycle engine according to the first and second embodiments (hereinafter also referred to simply as “lubricating oil composition”) are described together in detail unless otherwise specifically indicated. 
     [Base Oil] 
     The base oil (A) is properly selected from mineral oils and synthetic oils and used. 
     Examples of the mineral oil include a paraffinic mineral oil, a naphthenic mineral oil, an intermediate base mineral oil, and the like. For example, there are exemplified mineral oils obtained by refining a lubricating oil fraction which is obtained by vacuum distillation of an atmospheric residue obtained through atmospheric distillation of a crude oil, by performing at least one treatment among solvent deasphalting, solvent extraction, hydrogenolysis, solvent dewaxing, catalytic dewaxing, hydrorefining, and so on. More specifically, there are exemplified a light neutral oil, a medium neutral oil, a heavy neutral oil, and the like. 
     In addition, as the mineral oil, any mineral oils classified into Groups 1, 2, and 3 in the base oil category of API (The American Petroleum Institute) are usable; however, from the viewpoint of suppressing the generation of sludge, those classified into the Groups 2 and 3 are preferred. The base oil classified into the Group 1 is one in which a saturated component content is less than 90%, and/or a sulfur content is higher than 0.03%, and a viscosity index of which is 80 or more and less than 120. In addition, the base oil classified into the Group 2 is one in which a saturated component content is 90% or more, a sulfur content is 0.03% or less, and a viscosity index of which is 80 or more and less than 120. Furthermore, the base oil classified into the Group 3 is one in which a saturated component content is 90% or more, a sulfur content is 0.03% or less, and a viscosity index of which is 120 or more. 
     The sulfur content is a value as measured in conformity with JIS K2541, and the saturated component content is a value as measured in conformity with ASTM D2007. Furthermore, the viscosity index is a value as measured in conformity with JIS K2283. 
     Examples of the synthetic oil include a poly-α-olefin (PAO) that is a polymer of an α-olefin having 6 to 16 carbon atoms, an ester of every kind, such as a polyol ester, a dibasic acid ester, a phosphoric acid ester, etc., an ether of every kind, such as a polyphenyl ether, etc., a polyalkylene glycol, an alkylbenzene, an alkylnaphthalene, a base oil produced through isomerization of GTL WAX; and the like. 
     As the base oil (A), the mineral oil may be used alone, or may be used in combination of two or more thereof. In addition, the synthetic oil may be used alone, or may be used in combination of two or more thereof. Furthermore, a combination of one or more mineral oils and one or more synthetic oils may also be used. 
     In addition, the base oil (A) serves as a main component in the lubricating oil composition, and it is contained in an amount of typically 50% by mass or more, and preferably 60 to 97% by mass, and preferably 70 to 95% by mass on the basis of the total amount of the lubricating oil composition. 
     Although the viscosity of the base oil (A) is not particularly limited, it is preferably in a range of 2 to 25 mm 2 /s, more preferably in a range of 4 to 20 mm2/s, and still more preferably in a range of 5 to 15 mm 2 /s in terms of a kinematic viscosity at 100° C. As for the lubricating oil composition of the present embodiment, by making the kinematic viscosity of the base oil (A) relatively high in such a way, the lubricating oil composition is suitably usable for a four-stroke cycle engine to be operated under a high load, particularly a gas cogeneration system. 
     [Polyolefin (B)] 
     As the polyolefin (B), one having at least an isobutene-derived constituting unit and having a number average molecular weight of 500 to 10,000 is used. In view of the fact that the lubricating oil composition contains the component (B) having a number average molecular weight of 500 or more in this way, it is possible to increase the viscosity of the lubricating oil composition. 
     In the first embodiment, in view of the fact that the component (B) includes an isobutene-derived constituting unit and has a relatively low number average molecular weight as 10,000 or less, the generation of sludge in a high-temperature environment (for example, 200° C. or higher) is suppressed, and it is also possible to maintain the detergency (for example, piston detergency) satisfactory. Then, in cooperation with the matter that the sulfated ash content is low, adhesion of the metal component or stains onto the engine parts is suppressed. For that reason, even if cleaning of the engine parts is not frequently performed, a trouble, such as knocking, etc., is prevented from occurring. 
     In the second embodiment, in view of the fact that the component (B) includes an isobutene-derived constituting unit and has a relatively low number average molecular weight as 10,000 or less, it is possible to suppress the generation of sludge in a high-temperature environment (for example, 200° C. or higher). Then, by allowing the sulfated ash content to fall within a predetermined range as mentioned later, the degradation of oil at a high temperature is also suppressed, and it is easy to prevent the generation of turbo coking from occurring. 
     In the first and second embodiments, the number average molecular weight of the component (B) is preferably 600 to 6,000. When the number average molecular weight is 600 or more, it is easy to make the viscosity of the lubricating oil composition higher by the component (B). In addition, when the number average molecular weight is 6,000 or less, it is easy to prevent the generation of sludge more effectively from occurring. Then, for example, in the first embodiment, it is easy to make the detergency satisfactory. From these viewpoints, the number average molecular weight of the component (B) is more preferably 700 to 3,000. 
     As the olefin other than isobutene in the component (B), for example, an olefin having 2 to 20 carbon atoms is used. Specific examples thereof include ethylene, propylene, 1-butene, 2-butene, 3-methyl-1-butene, 4-phenyl-1-butene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3,3-dimethyl-1-pentene, 3,4-dimethyl-1-pentene, 4,4-dimethyl-1-pentene, 1-hexene, 4-methyl-1-hexene, 5-methyl-1-hexene, 6-phenyl-1-hexene, 1-octene, 1-decene, 1-dedecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, 1-eicosene, and the like. Among those, 1-butene is preferred. 
     The polyolefin includes the isobutene-derived constituting unit in an amount of preferably 50 to 100 mol %, more preferably 70 to 100 mol %, and still more preferably 80 to 100 mol %. In this way, when the amount of the isobutene-derived constituting unit is high, the generation of sludge is much more suppressed. Then, furthermore, in the first embodiment, it is easy to improve the detergency, and in the second embodiment, it is easy to improve the resistance to turbo coking. 
     In addition, from the viewpoints of suppressing the generation of sludge, and making it easy to improve the detergency in the first embodiment but the resistance to turbo coking in the second embodiment, the polyolefin (B) is preferably a polybutene obtained through polymerization of a butene of every kind, and above all, the polyolefin (B) is more preferably polyisobutene. The polyisobutene as referred to in the present specification includes not only one in which all of the constituting units are derived from only isobutene but also one in which a part of the constituting units (20 mol % or less, and preferably 10 mol % or less) is a constituting unit derived from other butene, such as 1-butene, etc. 
     The component (B) may be a hydrogenated product and may be, for example, a hydrogenated product of polybutene or a hydrogenated product of polyisobutene; however, the component (B) is preferably a non-hydrogenated material, namely, the aforementioned polybutene or polyisobutene is preferably a non-hydrogenated material. In the first and second embodiments, when the component (B) is a non-hydrogenated material, it is easy to more suppress the generation of sludge or the like. 
     In the respective embodiments, the content of the component (B) is 0.5 to 10% by mass on the basis of the total amount of the composition. When the content of the component (B) is 0.5% by mass or more, it is possible to appropriately improve the viscosity of the lubricating oil composition. In addition, when the content of the component (B) is 10% by mass or less, the generation of sludge or lowering of dispersibility is prevented from occurring. From these viewpoints, in the first embodiment, the content of the component (B) is more preferably 1.0 to 8.0% by mass, and still more preferably 1.5 to 7.0% by mass on the basis of the total amount of the composition. On the other hand, in the second embodiment, the content of the component (B) is more preferably 1.0 to 7.0% by mass, and still more preferably 1.5 to 5.0% by mass on the basis of the total amount of the composition. 
     [Metal-based Detergent] 
     It is preferred that the lubricating oil composition of each of the embodiments contains a metal-based detergent. Specifically, it is preferred that the lubricating oil composition contains an overbased calcium salicylate (C1) having a base number of 170 to 400 mgKOH/g and a neutral metal-based detergent (C2) selected from a neutral calcium sulfonate having a base number of 50 mgKOH/g or less and a neutral calcium salicylate having a base number of 50 mgKOH/g or less. 
     In the lubricating oil composition of each of the embodiments, while securing the base number maintaining properties by the component (C1) having a higher base number, the detergency is secured by also using the component (C2) having a lower base number together therewith, and thus, by using these two components it is possible to secure the high detergency while much more suppressing the generation of sludge. Furthermore, in the second embodiment, by regulating the contents of these components to desired contents as mentioned later, the high-temperature degradation of the lubricating oil composition is prevented, and it is easy to suppress the turbo coking. 
     In each of the embodiments, it is preferred to use, as the component (C2), the neutral calcium sulfonate among the foregoing. 
     In the first embodiment, the content of the overbased calcium salicylate (C1) is preferably 0.5 to 4.0% by mass, more preferably 0.75 to 3.0% by mass, and still more preferably 1.0 to 2.0% by mass on the basis of the total amount of the composition. In the first embodiment, by allowing the content of the component (C1) to fall within such a range, it is possible to make the base number of the lubricating oil composition high while regulating the later-mentioned sulfated ash content low. 
     In the first embodiment, the content of the neutral metal-based detergent (C2) is preferably 0.3 to 3% by mass, more preferably 0.5 to 2.5% by mass, and still more preferably 0.6 to 1.5% by mass on the basis of the total amount of the composition. In the first embodiment, by allowing the content of the component (C2) to fall within such a range, it is easy to secure the high detergency while regulating the sulfated ash content low. 
     In the second embodiment, the content of the overbased calcium salicylate (C1) is preferably 1.5 to 6.0% by mass, more preferably 2.0 to 5.0% by mass, and still more preferably 2.5 to 4.0% by mass on the basis of the total amount of the composition. In the second embodiment, by allowing the content of the component (C1) to fall within such a range, the base number of the lubricating oil composition is made high, thereby securing the long draining properties, and furthermore, it is easy to enhance the effect of suppressing the turbo coking. 
     In the second embodiment, the content of the neutral metal-based detergent (C2) is smaller than that of the aforementioned component (C1), and it is preferably 0.3 to 3% by mass, more preferably 0.5 to 2.0% by mass, and still more preferably 0.6 to 1.5% by mass on the basis of the total amount of the composition. In the second embodiment, by allowing the content of the component (C2) to fall within such a range, it is easy to secure the high detergency. In addition, by regulating the content of the component (C2) to 0.6% by mass or more, the effect for suppressing the turbo coking is enhanced. 
     In addition, the base number of the overbased calcium salicylate (C1) in each of the embodiments is 170 to 400 mgKOH/g as mentioned above; however, it is preferably 190 to 380 mgKOH/g, and more preferably 200 to 350 mgKOH/g. In the first embodiment, when the base number of the component (C1) falls within such a range, the generation of sludge is suppressed while making the detergency satisfactory. In addition, in the second embodiment, when the base number of the component (C1) falls within the aforementioned range, the base number of the lubricating oil composition can be made high, whereby not only it is easy to suppress the generation of sludge while making the detergency satisfactory, but also it is easy to enhance the effect for suppressing the turbo coking. 
     In addition, the base number of the component (C2) (namely, neutral calcium sulfonate and neutral calcium salicylate) in each of the embodiments is 50 mgKOH/g or less; however, it is preferably 10 to 40 mgKOH/g, and more preferably 15 to 30 mgKOH/g. In the first embodiment, by using a specified metal-based detergent having a low base number, it is easy to enhance the detergency of the lubricating oil composition. In addition, in the second embodiment, by using a predetermined amount of a specified metal-based detergent having a low base number, not only it is easy to enhance the detergency of the lubricating oil composition, but also it is easy to enhance the effect for suppressing the turbo coking. 
     In addition, when the aforementioned (C1) and (C2) components are blended, the resulting lubricating oil composition contains calcium. The calcium content in the lubricating oil composition in the first embodiment is preferably 500 to 1,800 ppm by mass. In the first embodiment, when the calcium content of the composition falls within such a range, it is possible to realize reduction of sludge and security of high detergency while suppressing the sulfated ash content. From such viewpoint, in the first embodiment, the calcium content in the lubricating oil composition is more preferably 700 to 1,600 ppm by mass, and still more preferably 900 to 1,500 ppm by mass. 
     It is preferred that the calcium content in the lubricating oil composition in the second embodiment is more than 1,800 ppm by mass and 3,500 ppm by mass or less. When the calcium content is more than 1,800 ppm by mass, the generation of sludge is suppressed while making the detergency satisfactory; and furthermore, the sulfated ash content becomes more than 0.7% by mass, whereby the effect of suppressing the turbo coking becomes excellent, and it is easy to secure the long draining properties. On the other hand, when the calcium content is 3,500 ppm by mass or less, the sulfated ash content is easy to become 1.2% by mass or less. From the foregoing viewpoints, the calcium content in the lubricating oil composition in the second embodiment is more preferably 2,000 to 3,000 ppm by mass, and still more preferably 2,100 to 2,900 ppm by mass. 
     As the overbased calcium salicylate (C1), there are exemplified those obtained by using a calcium salt of an alkylsalicylic acid, such as a monoalkylsalicylic acid, a dialkylsalicylic acid, etc., and overbasing the calcium salt. In addition, as the neutral calcium salicylate which is used as the component (C2), there are exemplified calcium salts of an alkylsalicylic acid, such as a monoalkylsalicylic acid, a dialkylsalicylic acid, etc. The alkyl group constituting the alkylsalicylic acid is a linear or branched alkyl group having preferably 4 to 30 carbon atoms, and more preferably 8 to 22 carbon atoms. 
     As the neutral calcium sulfonate which is used as the component (C2), calcium salts of a sulfonic acid of every kind can be used. Examples of the sulfonic acid which is used include an aromatic petroleum sulfonic acid, an alkylsulfonic acid, an arylsulfonic acid, an alkylarylsulfonic acid, and the like. Specifically, examples thereof may include dodecylbenzenesulfonic acid, dilaurylcetylbenzenesulfonic acid, paraffin wax-substituted benzenesulfonic acid, polyolefin-substituted benzenesulfonic acid, polyisobutylene-substituted benzenesulfonic acid, naphthalenesulfonic acid, and the like. 
     In the present specification, the base number of the metal-based detergent refers to one as measured by the perchloric acid method according to JIS K-2501. 
     [Ash-free Dispersant] 
     It is preferred that the lubricating oil composition of each of the embodiments further contains an ash-free dispersant. Although examples of the ash-free dispersant include a polybutenyl succinic acid imide (D1) and a boron-modified polybutenyl succinic acid imide (D2), it is preferred that the lubricating oil composition contains both of them. When the lubricating oil composition in the first embodiment contains these compounds, it is possible to improve both heat resistance and dispersibility without increasing the sulfated ash content, and for example, it is easy to suppress the generation of sludge to be caused due to thermal degradation. In addition, similarly, when the lubricating oil composition in the second embodiment contains these compounds, it is possible to improve both heat resistance and dispersibility. 
     Examples of the polybutenyl succinic acid imide (D1) include compounds represented by the following general formulae (1) and (2). 
     
       
         
         
             
             
         
       
     
     PIB in these general formulae (1) and (2) represents a polybutenyl group, and the number average molecular weight is typically 750 or more and 3,500 or less, and preferably 900 or more and 2,000 or less. When the number average molecular weight is 750 or more, the dispersibility is satisfactory, whereas when it is 3,500 or less, the  &lt; storage stability is satisfactory. In addition, n in each of the general formulae (1) and (2) is typically an integer of 1 to 5, and more preferably an integer of 2 to 4. 
     Although a production method of each of the compounds represented by these general formulae (1) and (2) is not particularly limited, the compounds can be produced by a known method. For example, the compounds can be obtained by allowing a polybutenyl succinic acid obtained through a reaction between polybutene and maleic anhydride at 100° C. or higher and 200° C. or lower to react with a polyamine, such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, etc. 
     Further, as the polybutenyl succinic acid imide (D1), a modified polybytenyl succinic acid imide obtained by allowing the compound represented by the general formula (1) or (2) to react with alcohol, aldehyde, ketone, alkylphenol, cyclic carbonate, epoxide, organic acid or the like may be used. 
     The boron-modified polybutenyl succinic acid imide (D2) is a compound obtained by allowing a boron compound to act on the aforementioned polybutenyl succinic acid imide (D1). 
     Examples of the boron compound include a boric acid, a boric acid salt, a boric acid ester, and the like. Examples of the boric acid include orthoboric acid, metaboric acid, paraboric acid, and the like. In addition, suitable examples of the boric acid salt include ammonium salts, such as ammonium borates, for example, ammonium metaborate, ammonium tetraborate, ammonium pentaborate, ammonium octaborate, etc., and the like. In addition, suitable examples of the boric acid ester include esters of a boric acid and an alkyl alcohol (desirably having 1 to 6 carbon atoms), such as monomethyl borate, dimethyl borate, trimethyl borate, monoethyl borate, diethyl borate, triethyl borate, monopropyl borate, dipropyl borate, tripropyl borate, monobutyl borate, dibutyl borate, tributyl borate, etc. 
     A total content of the polybutenyl succinic acid imide (D1) and the boron-modified polybutenyl succinic acid imide (D2) in each of the embodiments is preferably 0.01 to 0.15% by mass, and more preferably 0.02 to 0.12% by mass as expressed in terms of a nitrogen atom on the basis of the total amount of the lubricating oil composition. In addition, the aforementioned total content in the second embodiment is still more preferably 0.04 to 0.10% by mass. When the total content is such a lower limit value or more, the heat resistance and the dispersibility are satisfactory, and it is easy to prevent the generation of sludge or the like from occurring. In addition, when the total content is the upper limit value or less, the performances are easily exhibited corresponding to the content. 
     In addition, in each of the embodiments, in general, a mass ratio BIN of a boron content B to a nitrogen content N as derived from the polybutenyl succinic acid imide (D1) and the boron-modified polybutenyl succinic acid imide (D2) is preferably 0.05 to 1.5, and more preferably 0.15 to 1.2. 
     [Anti-wear Agent] 
     It is preferred that the lubricating oil composition of each of the embodiments contains a zinc dithiophosphate (E) as an anti-wear agent. As the zinc dithiophosphate (E), for example, a compound represented by the following general formula (3) is used. 
     
       
         
         
             
             
         
       
     
     In the general formula (3), R 1  to R 4  each independently represent a linear or branched alkyl group having 1 to 24 carbon atoms, a linear or branched alkenyl group having 2 to 24 carbon atoms, or an aralkyl group having 6 to 18 carbon atoms, and R 1  to R 4  may be the same as or different from each other. In the general formula (3), the carbon number of each of R 1  to R 4  is preferably 6 to 10 for the alkyl group and the alkenyl group, and 8 to 20 for aralkyl group, respectively. From the viewpoint of stability or the like, R 1  to R 4  are each preferably an alkyl group or an aralkyl group. 
     In the first embodiment, by using the zinc dithiophosphate, it is possible to improve wear resistance and oxidation stability in a small use amount thereof without excessively increasing the sulfated ash content. 
     In the second embodiment, by using the zinc dithiophosphate, it is possible to improve wear resistance in a small use amount thereof. In addition, high-temperature oxidation degradation or the like of the lubricating oil composition is also prevented from occurring, and it is easy to suppress the generation of turbo coking, too. 
     Examples of the alkyl group in R 1  to R 4  include a methyl group, an ethyl group, a n-propyl group, a n-butyl group, a n-pentyl group, a n-hexyl group, a n-heptyl group, a n-octyl group, a n-nonyl group, a n-decyl group, a n-undecyl group, a n-dodecyl group, a n-tridecyl group, a n-tetradecyl group, a n-pentadecyl group, a n-hexadecyl group, a n-heptadecyl group, a n-octadecyl group, n-nonadecyl group, a n-eicosyl group, a n-heneicosyl group, a n-docosyl group, n-tricosyl group, a n-tetracosyl group, and branched structural isomers of these alkyl groups. 
     In addition, examples of the alkenyl, group include a n-butenyl group, a n-pentenyl group, a n-hexenyl group, a n-heptenyl group, a n-octenyl group, a n-nonenyl group, a n-decenyl group, a n-undecenyl group, a n-dodecenyl group, a n-tridecenyl group, a n-tetradecenyl group, a n-pentadecenyl group, a n-hexadecenyl group, a n-heptadecenyl group, a n-octadecenyl group, a n-nonadecenyl group, a n-eicosenyl group, a n-heneicosenyl group, a n-docosenyl group, a n-tricosenyl group, a n-tetracosenyl group, and branched structural isomers of these alkenyl group. 
     Examples of the aralkyl group include an ethylphenyl group, a n-butylphenyl group, a n-propylphenyl group, a n-hexylphenyl group, a n-octylphenyl group, a n-nonylphenyl group, a n-decylphenyl group, a n-dodecylphenyl group, and branched structural isomers of these aralkyl groups. 
     In addition, R 1  to R 4  are each independently more preferably a primary alkyl group represented by a structure of R—CH 2 — (wherein R is an alkyl group having 1 to 23 carbon atoms). Above all, linear alkyl groups, such as a n-hexyl group, a n-heptyl group, a n-octyl group, a n-nonyl group, a n-decyl group, etc.; and branched alkyl groups, such as a 2-methylpropyl group, a 2-methylbutyl group, a 2-methylhexyl group, a 2-ethylhexyl group, etc., are still more preferred. In this way, when R 1  to R 4  are each a primary alkyl group or an aralkyl group, in the first and second embodiments, it is possible to more improve the oxidation stability of the lubricating oil composition. Furthermore, in the second embodiment, it is easy to suppress the generation of turbo coking. 
     The content of the zinc dithiophosphate (E) in the first embodiment is preferably 100 to 1,000 ppm by mass, more preferably 150 to 800 ppm by mass, and still more preferably 200 to 600 ppm by mass as expressed in terms of a phosphorus atom on the basis of the total amount of the lubricating oil composition. In the first embodiment, when the content of the component (E) falls within such a range, it is possible to make the wear resistance satisfactory while reducing the amount of the sulfated ash content. 
     The content of the zinc dithiophosphate (E) in the second embodiment is preferably 100 to 1,000 ppm by mass, more preferably 200 to 800 ppm by mass, and still more preferably 300 to 600 ppm by mass as expressed in terms of a phosphorus atom on the basis of the total amount of the lubricating oil composition. In the second embodiment, when the content of the component (E) is such a lower limit value or more, the wear resistance can be made satisfactory. In addition, when the content of the component (E) is the lower limit value or more, the high-temperature degradation is prevented from occurring, and it is easy to improve the resistance to turbo coking. On the other hand, when the content of the component (E) is the upper limit value or less, it is easy to exhibit the performances corresponding to the blending amount, and it is easy to regulate the sulfated ash content to 1.2% by mass or less. 
     [Antioxidant] 
     It is preferred that the lubricating oil composition of each of the embodiments contains, as an antioxidant, a hindered phenol compound (F1) and an alkyldiphenylamine compound (F2). 
     In the first embodiment, when the lubricating oil composition contains these two components as the antioxidant, the oxidation stability is enhanced, thereby making it possible to make the high-temperature detergency satisfactory. In addition, in the first embodiment, a sum total of the contents of the hindered phenol compound (F1) and the alkyldiphenylamine compound (F2) is preferably more than 1.5% by mass and 5.0% by mass or less on the basis of the total amount of the lubricating oil composition. When the sum total of the contents of the component (F1) and the component (F2) is more than 1.5% by mass, it is possible to sufficiently enhance the piston detergency. In addition, when the sum total of the contents of the component (F1) and the component (F2) is 5.0% by mass or less, the effects corresponding to the addition amount can be exhibited. 
     In the first embodiment, from the viewpoint of making the oxidation stability and the high-temperature detergency more satisfactory, the total sum of the contents of the aforementioned component (F1) and component (F2) is more preferably 2.0 to 4.5% by mass, and still more preferably 2.5 to 4.0% by mass. 
     In the second embodiment, a sum total of the contents of the hindered phenol compound (F1) and the alkyldiphenylamine compound (F2) is preferably more than 1.5% by mass and 6.0% by mass or less on the basis of the total amount of the lubricating oil composition. 
     As mentioned above, it is preferred that the lubricating oil for four-stroke cycle engine has high piston detergency, and in the second embodiment, it is also preferred to secure the high piston detergency. 
     In cooperation with the matter that the sludge is suppressed as mentioned above, when the lubricating oil composition of the second embodiment contains the antioxidants of the component (F1) and the component (F2) in an amount of more than 1.5% by mass in total, the oxidation stability is enhanced, thereby making it possible to secure the high piston detergency. In addition, when the total sum of the contents of the component (F1) and the component (F2) is 6.0% by mass or less, the effects corresponding to the addition amount can be exhibited. 
     In the second embodiment, from the viewpoint of making the oxidation stability and the piston detergency more satisfactory, the sum total of the contents of the aforementioned component (F1) and component (F2) is more preferably 2.0 to 5.5% by mass, and still more preferably 2.5 to 5.0% by mass. 
     In the first and second embodiments, though a mass ratio (F2/F1) of the alkyldiphenylamine compound (F2) to the hindered phenol compound (F1) is not particularly limited, it is preferably about 1/3 to 3/1, and more preferably about 1/2 to 2/1. 
     Representatively, as the hindered phenol compound (F1), those in which a t-butyl group is substituted on the ortho-position against the phenolic hydroxyl group are exemplified, and examples thereof include bisphenol-based compounds, monophenol-based compounds, and the like. Examples of the bisphenol-based compound include 4,4′-methylenebis(2,6-di-t-butylphenol), 4,4′-bis(2,6-di-t-butylphenol), 4,4′-isopropylidenebis(2,6-di-t-butylphenol), bis(3,5-di-t-butyl-4-hydroxybenzyl)sulfide, thiodiethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 4,4′-bis(2-methyl-6-t-butylphenol), 2,2-methylenebis(4-ethyl-6-t-butylphenol), 2,2′-methylenebis(4-methyl-6-t-butylphenol), 4,4′-butylidenebis(3-methyl-6-t-butylphenol), 4,4′-thiobis(2-methyl-6-t-butylphenol), 4,4′-thiobis(3-methyl-6-t-butylphenol), 2,2′-thiobis(4-methyl-6-t-butylphenol), bis(3-methyl-4-hydroxy-5-t-butylbenzyl)sulfide, and the like. 
     In addition, examples of the monophenol-based compound include 2,6-di-t-butyl-4-alkylphenols in which an alkyl moiety thereof has 1 to 4 carbon atoms, such as 2,6-d-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, etc.; alkyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionates in which an alkyl moiety thereof has 4 to 12 carbon atoms, such as n-octyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, 6-methylheptyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, etc.; and 2,4-dimethyl-6-t-butylphenol. 
     Among those, from the viewpoint of making the oxidation stability and the high-temperature detergency more satisfactory, those in which a t-butyl group is substituted on both of the ortho-positions against the phenolic hydroxyl group are preferred. 
     Examples of the alkyldiphenylamine compound (F2) include monoalkyl-substituted diphenylamines in which an alkyl group having 3 to 10 carbon atoms is substituted on a one-sided phenyl group, such as mono-t-butyldiphenylamine, monooctyldiphenylamine, monononyldiphenylamine, etc.; dialkyldiphenylamines in which each alkyl group has 3 to 10 carbon atoms, such as 4,4′-dibutyldiphenylamine, 4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine, 4,4′-diheptyldiphenylamine, 4,4′-dioctyldiphenylamine, 4,4′-dinonyldiphenylamine, 4-butyl-4′-octyldiphenylamine, etc.; polyalkyldiphenylamines having 3 or more alkyl groups, in which each alkyl group has 1 to 10 carbon atoms, such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, tetranonyldiphenylamine, di(2,4-diethylphenyl)amine, di(2-ethyl-4-nonylphenyl)amine, etc.; and the like. Among those, dialkyldiphenylamines in which each alkyl group has 3 to 10 carbon atoms are preferred. 
     [Other Additives] 
     The lubricating oil composition may further contain other additives, such as a pour-point depressant, a metal deactivator, a demulsifier, an antifoaming agent, etc. 
     Examples of the pour-point depressant include a polymethacrylate, a polyacrylate, and the like. Examples of the metal deactivator include a benzotriazole-based compound, a tolyltriazole-based compound, a thiadiazole-based compound, an imidazole-based compound, a pyrimidine-based compound, and the like. 
     As the demulsifier, a surfactant is used. Examples thereof include polyalkylene glycol-based nonionic surfactants, such as a polyoxyethylene alkyl ether, a polyoxypropylene alkyl ether, a polyoxyethylene alkyl phenyl ether, a polyoxypropylene alkyl phenyl ether, a polyoxyethylene alkyl naphthyl ether, etc.; and the like. 
     In addition, examples of the antifoaming agent include a silicone oil, a fluorosilicone oil, a fluoroalkyl ether, and the like. 
     [Lubricating Oil Composition] 
     As mentioned above, the lubricating oil composition in the first embodiment has the sulfated ash content of 0.3 to 0.7% by mass. In the first embodiment, by using the component (B), the generation of sludge is suppressed, and the detergency can also be maintained satisfactory. Therefore, in cooperation with the matter that the sulfated ash content is low as 0.7% by mass or less, adhesion of the metal component or stains onto the engine parts is suppressed. For that reason, even if cleaning of the engine parts is not frequently performed, the generation of a trouble, such as knocking, etc., is prevented from occurring. 
     In addition, in the first embodiment, by regulating the sulfated ash content to 0.3% by mass or more, the metal-based additive is used in an amount to some extent. For that reason, as mentioned above, it is easy to obtain the effects, such as suppression of sludge, security of high detergency, improvement of wear resistance, etc., due to the action of the metal-based detergent, the anti-wear agent, or the like. 
     From the foregoing viewpoints, the sulfated ash of the lubricant oil composition of the first embodiment is preferably 0.35 to 0.65% by mass, and more preferably 0.40 to 0.60% by mass. 
     As mentioned above, the lubricating oil composition in the second embodiment has the sulfated ash content of more than 0.7% by mass and 1.2% by mass or less. In the second embodiment, by regulating the sulfated ash content to more than 0.7% by mass, since the amount of the metal component derived from the metal-based detergent or zinc dithiophosphate in the composition increases, in cooperation with the matter that the aforementioned component (B) is used, the high-temperature degradation is suppressed, the turbo coking is hardly generated in the lubricating oil composition, and furthermore, the long draining properties are easily secured, too. 
     In addition, by regulating the sulfated ash content to 1.2% by mass or less, since the amount of the metal in the composition does not become high more than necessity, deposition of the metal component in a combustion chamber or the like is prevented from occurring. In the lubricating oil composition of the second embodiment, the long draining properties can be realized, and therefore, it is possible to operate the engine over a long period of time without replacing the lubricating oil composition. Then, even when the lubricating oil composition of the second embodiment is used over a long period of time without being replaced, the metal component is hardly deposited as mentioned above, and therefore, knocking or the like is not generated before replacement. 
     From the foregoing viewpoints, the sulfated ash content of the lubricating oil composition of the second embodiment is preferably 0.80 to 1.15% by mass, and more preferably 0.90 to 1.10% by mass. 
     With respect to the lubricating oil composition of each of the embodiments, the kinematic viscosity at 100° C. is preferably 6.9 mm 2 /s or more and less than 21.9 mm 2 /s, more preferably 9.3 mm 2 /s or more and less than 18.0 mm 2 /s, and still more preferably 9.3 mm 2 /s or more and less than 16.3 mm 2 /s. By allowing the kinematic viscosity at 100° C. to fall within such a range, the lubricating oil composition of each of the first and second embodiments is suitably usable for an engine to be used under a high load, particularly a gas cogeneration system. 
     The lubricating oil composition of each of the embodiments is used as an engine oil performing lubrication between respective parts in a four-stroke cycle engine. 
     In addition, the lubricating oil composition of the first embodiment is suitably used for a four-stroke cycle engine to be continuously operated under a high load in shipping or fixed-type power generation equipment or the like. Specifically, examples of such a four-stroke cycle engine include those in which a maximum power is 200 kW or more, an operation at 60% or more of the maximum power is continued for 10 hours or more, and an oil replacement cycle is 500 hours or more. The term “hours” referred to in the oil replacement cycle means a sum total of times for which the engine is operated. 
     Even when the lubricating oil composition of the first embodiment is used such a four-stroke cycle engine to be continuously operated under a high load, the piston detergency is satisfactory, the generation of sludge is suppressed, and knocking or the like generated due to the matter that the metal component or stains adhere to the engine parts is also prevented from occurring. In consequence, it is possible to reduce a cleaning work of the engine parts to be performed at the time of oil replacement, whereby the time for maintenance is reduced. 
     The lubricating oil composition of the second embodiment is suitably used for a four-stroke cycle engine equipped with a turbocharger, which is continuously operated under a high load in shipping or fixed-type power generation equipment or the like. Specifically, examples of such a four-stroke cycle engine include those in which a maximum power is 200 kW or more, an operation at 60% or more of the maximum power is continued for 10 hours or more, and an oil replacement cycle is 1,000 hours or more. 
     Even when the lubricating oil composition of the second embodiment is used for a four-stroke cycle engine to be continuously operated under a high load as mentioned above, the piston detergency is satisfactory, and the generation of sludge is suppressed. Furthermore, the high-temperature degradation is prevented from occurring, the effect of suppressing the turbo coking is satisfactory, and the long draining properties are secured. 
     The lubricating oil compositions of the first and second embodiments are suitably used for a gas engine, and in particular, are suitably used for a gas cogeneration system. 
     [Production Method of Lubricating Oil Composition for Four-Stroke Cycle Engine] 
     A production method of a lubricating oil composition for four-stroke cycle engine in the first embodiment includes blending a base oil (A) with a polyolefin (B) and regulating a sulfated ash content to 0.3 to 0.7% by mass. 
     Furthermore, a production method of a lubricating oil composition for four-stroke cycle engine in the second embodiment includes blending a base oil (A) with a polyolefin (B) and regulating a sulfated ash content to more than 0.7% by mass and 1.2% by mass or less. 
     In the aforementioned respective production methods, details of the base oil (A) and the polyolefin (B) are those as mentioned above; and the amount (blending amount) at which each of the components is blended is the same as the aforementioned content, and its explanation is omitted. 
     In addition, the aforementioned metal-based detergent, ash-free dispersant, anti-wear agent, antioxidant, and other additives are optionally blended in the same contents as the aforementioned contents, and are regulated such that the sulfated ash content is 0.3 to 1.2% by mass (0.3 to 0.7% by mass in the first embodiment; and more than 0.7% by mass and 1.2% by mass or less in the second embodiment). Details of the lubricating oil compositions obtained by the present methods are the same as those mentioned above, and therefore, explanations thereof are omitted. 
     In the present production methods, each of the components may be blended in the base oil by any method, and the technique and blending order thereof are not restricted. 
     EXAMPLES 
     Next, the present invention is hereunder described in more detail by reference to the Examples, but it should be construed that the present invention is by no means limited by these Examples. 
     The respective properties are those obtained according to the following measuring methods. 
     (1) Kinematic Viscosity: 
     The kinematic viscosity is a value as measured using a glass-made capillary viscometer in conformity with JIS K2283-2000. 
     (2) Sulfur Content: 
     The sulfur content was measured in conformity with JIS K2541. 
     (3) Base Number: 
     The base number of each of the metal detergent and the lubricating oil composition is one as measured by the perchloric acid method in conformity with JIS K2501-2003. 
     (4) Sulfated Ash Content: 
     The sulfated ash content was measured in conformity with JIS K2272. 
     (5) Number Average Molecular Weight and Weight Average Molecular Weight: 
     The number average molecular weight and the weight average molecular weight of each component were measured using a system of HLC-8220 model, manufactured by Tosoh Corporation, installed with two columns of TSKgel GMH-XL and one column of G2000H-XL, all of which are manufactured by Tosoh 
     Corporation, under conditions of a detector: refractive index detector, a measurement temperature: 40° C., a mobile phase: tetrahydrofuran, a flow rate: 1.0 mL/min, and a concentration: 1.0 mg/mL and determined as expressed in terms of standard polystyrene. 
     (6) Calcium Content: 
     The calcium content in the lubricating oil composition was measured in conformity with JPI-5S-38-03. 
     (7) Hot Tube Test: 
     The hot tube test was carried out at a measurement temperature of 310° C. in conformity with JPI-5S-55-99. A lacquer adhered onto the inside of the glass tube was compared with a color sample. When the lacquer assumed transparent, it was rated as a score of 10, whereas when the lacquer assumed black, it was rated as a score of 0. Also, the mass of the lacquer adhered onto the inside of the glass tube was measured. The higher the score, or the smaller the amount of the lacquer, the higher the performance of the sample is. 
     (8) Panel Coking Test: 
     In conformity with the Fed. Test Method Std. 791-3462, the test was performed under a condition at a panel temperature of 300° C., 310° C., or 320° C. and an oil temperature of 100° C., and in a cycle of a splash time of 15 seconds and a cessation time of 45 seconds, for 3 hours. After completion of the test, the coked substance adhered onto the panel was evaluated. 
     (9) High-Temperature Degradation Test (Test of Resistance to Turbo Coking): 
     0.5 g of a test oil was put into an iron-made dish-shaped vessel having a diameter of 35 mm and a height of 3 mm and heated at 250° C. for 6 hours. Thereafter, the state of a residue was observed and evaluated according to the following evaluation criteria. 
     A: Flowable, B: Non-flowable 
     Examples 1 to 5 and Comparative Examples 1 to 4 
     Lubricating oil compositions of the Examples and Comparative Examples, each having a formulation shown in Table 1, were prepared and measured for properties of each of the lubricating oil compositions. In addition, the lubricating oil composition of each of the Examples and Comparative Examples was evaluated by the hot tube test and the panel coking test. The results are shown in Table 1. The panel coking test was performed at a panel temperature of 300° C. and 320° C., respectively. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Example 
                 Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Formulation  
                 Base oil 
                 mass % 
                 82.37 
                 82.37 
                 84.87 
                 84.47 
                 85.37 
                 86.37 
                 77.37 
                 84.92 
                 82.37 
               
               
                 of 
                 Polybutene-1 
                 mass % 
                 5.00 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 lubricating  
                 Polybutene-2 
                 mass % 
                   
                 5.00 
                   
                   
                   
                   
                   
                   
                   
               
               
                 oil 
                 Polybutene-3 
                 mass % 
                   
                   
                 2.50 
                 2.50 
                   
                   
                   
                   
                   
               
               
                 composition 
                 Polybutene-4 
                 mass % 
                   
                   
                   
                   
                 2.00 
                   
                   
                   
                   
               
               
                   
                 Polybutene-5 
                 mass % 
                   
                   
                   
                   
                   
                 1.00 
                   
                   
                   
               
               
                   
                 Bright stock 
                 mass % 
                   
                   
                   
                   
                   
                   
                 10.00 
                   
                   
               
               
                   
                 OCP-1 
                 mass % 
                   
                   
                   
                   
                   
                   
                   
                 2.45 
                   
               
               
                   
                 OCP-2 
                 mass % 
                   
                   
                   
                   
                   
                   
                   
                   
                 5.00 
               
               
                   
                 Metal-based detergent A 
                 mass % 
                 1.45 
                 1.45 
                 1.45 
                 1.45 
                 1.45 
                 1.45 
                 1.45 
                 1.45 
                 1.45 
               
               
                   
                 Metal-based detergent B 
                 mass % 
                 0.60 
                 0.60 
                 0.60 
                 1.00 
                 0.60 
                 0.60 
                 0.60 
                 0.60 
                 0.60 
               
               
                   
                 Ash-free dispersant A 
                 mass % 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
               
               
                   
                 Ash-free dispersant B 
                 mass % 
                 2.20 
                 2.20 
                 2.20 
                 2.20 
                 2.20 
                 2.20 
                 2.20 
                 2.20 
                 2.20 
               
               
                   
                 Anti-wear agent 
                 mass % 
                 0.58 
                 0.58 
                 0.58 
                 0.58 
                 0.58 
                 0.58 
                 0.58 
                 0.58 
                 0.58 
               
               
                   
                 Antioxidant 
                 mass % 
                 3.00 
                 3.00 
                 3.00 
                 3.00 
                 3.00 
                 3.00 
                 3.00 
                 3.00 
                 3.00 
               
               
                   
                 Other additives 
                 mass % 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
               
               
                   
                 Total 
                 mass % 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Sum total of ash-free dispersants  
                 mass % 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
               
               
                 (expressed in terms of nitrogen atom) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Ash-free dispersant (B/N ratio) 
                   
                 0.43 
                 0.43 
                 0.43 
                 0.43 
                 0.43 
                 0.43 
                 0.43 
                 0.43 
                 0.43 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Properties  
                 Kinematic viscosity  
                 mm 2 /s 
                 138.8 
                 138.0 
                 133.6 
                 132.5 
                 134.1 
                 132.2 
                 135.0 
                 130.4 
                 125.8 
               
               
                 of 
                 at 40° C.  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 lubricating  
                 Kinematic viscosity  
                 mm 2 /s 
                 14.87 
                 14.73 
                 14.50 
                 14.35 
                 14.46 
                 14.56 
                 14.63 
                 15.01 
                 14.54 
               
               
                 oil 
                 at 100° C. 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 composition 
                 Calcium content 
                 ppm 
                 1280 
                 1280 
                 1270 
                 1360 
                 1290 
                 1280 
                 1260 
                 1280 
                 1270 
               
               
                   
                 Phosphorus content 
                 ppm 
                 420 
                 420 
                 418 
                 415 
                 418 
                 420 
                 419 
                 416 
                 423 
               
               
                   
                 Base number  
                 mgKOH/ 
                 7.62 
                 7.40 
                 7.38 
                 7.30 
                 7.52 
                 7.45 
                 7.46 
                 7.65 
                 7.47 
               
               
                   
                 (perchloric acid method) 
                 g 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Sulfated ash content 
                 mass % 
                 0.52 
                 0.51 
                 0.49 
                 0.54 
                 0.50 
                 0.50 
                 0.49 
                 0.51 
                 0.48 
               
               
                 Evaluation 
                 Hot tube test at 310° C. :  
                 — 
                 9.5 
                 9.5 
                 9.0 
                 9.5 
                 9.0 
                 9.0 
                 9.5 
                 8.0 
                 7.5 
               
               
                 results 
                 Merit score 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Hot tube test: Deposit 
                 mg 
                 0.1 
                 0.1 
                 0.3 
                 0.1 
                 0.2 
                 0.2 
                 0.1 
                 0.4 
                 0.5 
               
               
                   
                 Panel coking test: Oil  
                 mg 
                 8 
                 7 
                 12 
                 6 
                 27 
                 24 
                 12 
                 50 
                 45 
               
               
                   
                 temperature at 100° C. / 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 panel temperature at  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 300° C. : Deposit 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Panel coking test: Oil  
                 mg 
                 65 
                 75 
                 78 
                 48 
                 80 
                 125 
                 120 
                 141 
                 96 
               
               
                   
                 temperature at 100° C. / 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 panel temperature at  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 320° C. : Deposit 
               
               
                   
               
               
                 * The calcium content and the phosphorus content are a calcium content and a phosphorus content, respectively in the lubricating oil composition and expressed on the basis of the total amount of the composition. 
               
               
                 * The respective components in Table 1 are as follows. 
               
               
                 Base oil: 500N mineral oil (hydrorefined mineral oil), kinematic viscosity at 100° C. : 10.9 mm 2 /s, sulfur content: 0.01% by mass or less, API Classification: Group 2 
               
               
                 Polybutene-1 Polyisobutene, number average molecular weight: 850, non-hydrogenated 
               
               
                 Polybutene-2 Polyisobutene, number average molecular weight: 950, non-hydrogenated 
               
               
                 Polybutene-3 Polyisobutene, number average molecular weight: 1,800, non-hydrogenated 
               
               
                 Polybutene-4 Polyisobutene, number average molecular weight: 4,500, hydrogenated 
               
               
                 Polybutene-5 Polyisobutene, number average molecular weight: 30,000, non-hydrogenated 
               
               
                 Bright stock 150BS mineral oil (hydrorefined mineral oil), kinematic viscosity at 100° C. : 33.3 mm 2 /s, sulfur content: 0.01% by mass or less 
               
               
                 OCP-1 Ethylene-propylene copolymer, weight average molecular weight: 91,000 
               
               
                 OCP-2 Styrene/isoprene block copolymer, weight average molecular weight: 132,500 
               
               
                 Metal-based detergent A Overbased calcium salicylate, calcium content: 7.8% by mass, base number: 225 mgKOH/g 
               
               
                 Metal-based detergent B Calcium sulfonate, calcium content: 2.35% by mass, base number: 17 mgKOH/g 
               
               
                 Ash-free dispersant A Polybutenyl succinic acid imide, nitrogen content: 1.0% by mass 
               
               
                 Ash-free dispersant B Boron-modified polybutenyl succinic acid imide, nitrogen content: 1.23% by mass, boron content: 1.3% by mass 
               
               
                 Anti-wear agent Zinc di-2-ethylhexyldithiophosphate, phosphorus content: 7.40% by mass, zinc content: 8.9% by mass 
               
               
                 Antioxidant Mixture of antioxidant F1 and antioxidant F2 (F2/F1 (mass ratio) = 2/1.5) 
               
               
                 Antioxidant F1 Mixture of alkyl-3-(4-hydroxy-3,5-di-t-buthylphenyl) propionates (wherein the alkyl moiety is a mixture of alkyls having 7 to 9 carbon atoms) 
               
               
                 Antioxidant F2 Dioctyldiphenylamine 
               
               
                 Other additives Pour-point depressant, metal deactivator, antifoaming agent, and demulsifier 
               
            
           
         
       
     
     In the light of the above, in Examples 1 to 5, since the polybutene having a predetermined molecular weight was blended, the results of the hot tube test and the panel coking test were satisfactory while regulating the kinematic viscosity to an appropriate value; and not only the detergency was excellent, but also the generation of sludge could be prevented from occurring. On the other hand, in Comparative Examples 1 to 4, since the bright stock or OCP was used in place of the polybutene, or a polybutene having a large molecular weight was used, the results in at least one of the hot tube test and the panel coking test were worse, and the generation of sludge could not be prevented from occurring with making the detergency satisfactory. 
     Examples 6 to 9 and Comparative Examples 5 to 7 
     Lubricating oil compositions of the Examples and Comparative Examples, each having a formulation shown in Table 2, were prepared and measured for properties of each of the lubricating oil compositions. In addition, the lubricating oil composition of each of the Examples and Comparative Examples was evaluated by the panel coking test and the high-temperature degradation test. The results are shown in Table 2. The panel coking test was performed at a panel temperature of 300° C. and 310° C., respectively. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Example 
                 Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 6 
                 7 
                 8 
                 9 
                 5 
                 6 
                 7 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Formulation of 
                 Base oil 
                 mass % 
                 82.75 
                 82.25 
                 81.85 
                 83.25 
                 75.25 
                 81.95 
                 84.25 
               
               
                 lubricating oil 
                 Polybutene-3 
                 mass % 
                 2.50 
                   
                   
                   
                   
                   
                   
               
               
                 composition 
                 Polybutene-6 
                 mass % 
                   
                 3.00 
                 3.00 
                 3.00 
                   
                   
                   
               
               
                   
                 Polybutene-5 
                 mass % 
                   
                   
                   
                   
                   
                   
                 1.00 
               
               
                   
                 Bright stock 
                 mass % 
                   
                   
                   
                   
                 10.00 
                   
                   
               
               
                   
                 OCP-1 
                 mass % 
                   
                   
                   
                   
                   
                 3.30 
                   
               
               
                   
                 Metal-based detergent A 
                 mass % 
                 3.15 
                 3.15 
                 3.15 
                 3.15 
                 3.15 
                 3.15 
                 3.15 
               
               
                   
                 Metal-based detergent B 
                 mass % 
                 0.60 
                 0.60 
                 1.00 
                 0.60 
                 0.60 
                 0.60 
                 0.60 
               
               
                   
                 Ash-free dispersant A 
                 mass % 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
                 4.00 
               
               
                   
                 Ash-free dispersant B 
                 mass % 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
               
               
                   
                 Anti-wear agent 
                 mass % 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
               
               
                   
                 Antioxidant 
                 mass % 
                 3.50 
                 3.50 
                 3.50 
                 2.50 
                 3.50 
                 3.50 
                 3.50 
               
               
                   
                 Other additives 
                 mass % 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
                 0.80 
               
               
                   
                 Total 
                 mass % 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sum total of ash-free dispersants  
                 mass % 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
               
               
                 (expressed in terms of nitrogen atom) 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Ash-free dispersant (B/N ratio) 
                   
                 0.40 
                 0.40 
                 0.40 
                 0.40 
                 0.40 
                 0.40 
                 0.40 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Properties of 
                 Kinematic viscosity at 40° C.  
                 mm 2 /s 
                 129.1 
                 138.0 
                 138.0 
                 138.6 
                 135.0 
                 130.4 
                 125.2 
               
               
                 lubricating oil 
                 Kinematic viscosity at 100° C.  
                 mm 2 /s 
                 14.30 
                 14.73 
                 14.73 
                 14.80 
                 14.63 
                 15.01 
                 14.03 
               
               
                 composition 
                 Calcium content 
                 ppm 
                 2550 
                 2580 
                 2690 
                 2670 
                 2600 
                 2560 
                 2580 
               
               
                   
                 Phosphorus content 
                 ppm 
                 519 
                 508 
                 520 
                 516 
                 518 
                 517 
                 519 
               
               
                   
                 Base number (perchloric acid method) 
                 mgKOH/g 
                 12.0 
                 12.2 
                 12.2 
                 12.1 
                 12.2 
                 12.3 
                 12.1 
               
               
                   
                 Sulfated ash content 
                 mass % 
                 0.99 
                 1.00 
                 1.04 
                 1.02 
                 0.99 
                 1.00 
                 0.97 
               
               
                 Evaluation 
                 Panel coking test: Oil temperature at  
                 mg 
                 7 
                 12 
                 5 
                 15 
                 53 
                 50 
                 35 
               
               
                 results 
                 100° C. /panel temperature at  
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 300° C. : Deposit 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Panel coking test: Oil temperature at  
                 mg 
                 35 
                 40 
                 28 
                 52 
                 94 
                 141 
                 75 
               
               
                   
                 100° C. /panel temperature at  
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 310° C. : Deposit 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 High-temperature degradation test 
                 Visual 
                 A 
                 A 
                 A 
                 A 
                 B 
                 B 
                 B 
               
               
                   
                   
                 inspection 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
               
                 * The calcium content and the phosphorus content are a calcium content and a phosphorus content, respectively in the lubricating oil composition and expressed on the basis of the total amount of the composition. 
               
            
           
         
       
     
     With respect to each of the components in Table 2, Polybutent-6 is as follows, and others are the same as those mentioned above. 
     Polybutene-6: Polyisobutene, number average molecular weight: 750, non-hydrogenated 
     In the light of the above, in Examples 6 to 9, since the polybutene having a predetermined molecular weight was blended and the various additives were blended such that the sulfated ash content was more than 0.7% by mass, the results of the panel coking test were satisfactory and thus the generation of sludge could be prevented from occurring, while regulating the kinematic viscosity to an appropriate value. Furthermore, the results of the high-temperature degradation test were satisfactory, and thus the resistance to turbo coking was satisfactory. In this way, in the present Examples, in the lubricating oil composition with a high sulfated ash content, the generation of sludge is prevented from occurring, and the resistance to turbo coking is satisfactory, and thus, it is easy to secure the long draining properties. 
     On the other hand, in Comparative Examples 5 to 7, since the bright stock or OCP was used in place of the polybutene, or a polybutene having a large molecular weight was used, the results of the panel coking test were worse, and the generation of sludge could not be prevented from occurring while making the detergency satisfactory. Furthermore, the results of the high-temperature degradation test were not satisfactory also, and the effect of suppressing the turbo coking was low. 
     Furthermore, with respect to Examples 6 to 9, the hot tube test was also carried out. The test results are shown below. As shown in Table 3, in Examples 6 to 9, the results of the hot tube test were satisfactory, and the piston detergency was also satisfactory. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Example 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 6 
                 7 
                 8 
                 9 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Merit score at 310° C. 
                 — 
                 9.5 
                 9.5 
                 9.5 
                 9.0 
               
               
                   
                 Deposit 
                 mg 
                 0.1 
                 0.1 
                 0.0 
                 0.2