Patent Publication Number: US-2017369808-A1

Title: Lubricating oil composition for internal combustion engine

Description:
FIELD 
     The present invention relates to lubricating oil compositions for internal combustion engines. 
     BACKGROUND 
     Recent years, it has been proposed to replace conventional naturally aspirated engines with smaller displacement engines having turbochargers (turbocharged downsized engines) for the purpose of reducing fuel consumption of internal combustion engines for automobiles, especially of gasoline engines for automobiles. Using turbocharged downsized engines with turbochargers makes it possible to reduce displacement while output power is kept, and to achieve low fuel consumption. 
     On the other hand, a phenomenon that a cylinder has ignition prior to predetermined timing (LSPI: Low Speed Pre-Ignition) may occur in a turbocharged downsized engine when torque is increased in a low rotation speed range. LSPI causes more energy loss, which limits improvement of fuel consumption and increase of low speed torque. Engine oil is suspected of influencing the occurrence of LSPI. These days, engine oil including Ca-based detergent and Mg-based detergent together is proposed in order to secure detergency while controlling LSPI. However, the fuel efficiency of such engine oil is not ensured yet or is insufficient. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2011-140572 A 
     Patent Literature 2: JP 2013-159734 A 
     Patent Literature 3: WO 2015/114920 A1 
     Patent Literature 4: WO 2015/171292 A1 
     Patent Literature 5: WO 2015/171978 A1 
     Patent Literature 6: WO 2015/171980 A1 
     Patent Literature 7: WO 2015/171981 A1 
     SUMMARY 
     Technical Problem 
     An object of the present invention is to provide a lubricating oil composition for an internal combustion engine that can secure LSPI suppression performance and detergency, and improve fuel efficiency at the same time. 
     Solution to Problem 
     A lubricating oil composition for an internal combustion engine of the present invention comprises: (A) a lubricant base oil having kinematic viscosity at 100° C. of 2 to 5 mm 2 /s; (B) a metallic detergent in an amount of 500 to 2500 mass ppm in terms of calcium and 100 to 1000 mass ppm in terms of magnesium, on the basis of the total mass of the composition, the metallic detergent comprising both (B1) a calcium-containing metallic detergent and (B2) a magnesium-containing metallic detergent; (C) a boron-containing additive in an amount of 50 to 1000 mass ppm in terms of boron on the basis of the total mass of the composition, wherein the boron-containing additive is oil-soluble or oil-dispersible and is stable in oil, and wherein the boron-containing additive may compose at least a part of the component (B); and (D) an oil-soluble organic molybdenum compound in an amount of 100 to 2000 mass ppm in terms of molybdenum on the basis of the total mass of the composition, wherein a mass ratio (MB/Mg) of boron content of the composition (MB) to magnesium content of the composition (Mg) is 0.5 to 10; and the composition satisfies one or more requirement selected from the following (i) to (iii): 
     (i) the boron content of the composition is no less than 270 mass ppm on the basis of the total mass of the composition; 
     (ii) the component (C) comprises a boric acid salt-overbased metallic detergent, wherein the boric acid salt-overbased metallic detergent may compose at least a part of the component (B1) or the component (B2) or the combination thereof; and 
     (iii) the mass ratio (MB/Mg) of the boron content of the composition (MB) to the magnesium content of the composition (Mg) is no less than 0.8. 
     In this specification, “kinematic viscosity at 100° C.” means kinematic viscosity at 100° C., which is specified by ASTM D-445; and “HTHS viscosity at 150° C.” means viscosity at a high shear rate and high temperature at 150° C., which is specified by ASTM D4683. 
     Concerning the component (C), that the boron-containing additive “may compose at least a part of the component (B)” means that this boron-containing additive may either compose at least a part of the component (B), or be an additive that is not the component (B). 
     Concerning the requirement (ii), that the boric acid salt-overbased metallic detergent “may compose at least a part of the component (B1) or the component (B2) or the combination thereof” means that this metallic detergent may compose at least a part of the component (B1), compose at least a part of the component (B2), compose at least a part of the component (B1) and at least a part of the component (B2), or be a metallic detergent that is not the component (B1) or the component (B2). When the requirement (ii) is satisfied, the boric acid salt-overbased metallic detergent contributes to both the content of the component (C) and the content of the component (B). 
     When the requirement (iii) is satisfied, the range of the mass ratio of the composition MB/Mg corresponds to a logical product of the range 0.5 to 10 and the range specified in the requirement (iii) “no less than 0.8”, that is, 0.8 to 10. 
     Advantageous Effects of Invention 
     The lubricating oil composition for an internal combustion engine of the present invention makes it possible to secure LSPI suppression performance and detergency, and to improve fuel efficiency at the same time. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The present invention will be described hereinafter. Expression “A to B” concerning numeral values A and B means “no less than A and no more than B” unless otherwise specified. In such expression, if a unit is added only to the numeral value B, the same unit is applied to the numeral value A as well. A word “or” means a logical sum unless otherwise specified. 
     &lt;(A) Lubricant Base Oil&gt; 
     In the lubricating oil composition of the present invention, a lubricant base oil of 2 to 5 mm 2 /s in kinematic viscosity at 100° C. (hereinafter may be referred to as “lubricant base oil according to this embodiment”) is used as a base oil. 
     Examples of the lubricant base oil according to this embodiment include paraffinic mineral oils, paraffinic base oils, isoparaffinic base oils, and mixtures thereof having a kinematic viscosity at 100° C. of 2 to 5 mm 2 /s, which are obtained by refining lubricant oil fractions that are obtained by atmospheric distillation and/or vacuum distillation of crude oils, through one or more refining processes selected from solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, clay treatment, etc. 
     Preferred examples of the lubricant base oil according to this embodiment include a base oil, a row material of which is any of the following base oils (1) to (8), and which is obtained by recovering lubricant oil fractions derived from refining, through a predetermined refining method, oil of the row material and/or lubricant oil fractions recovered from the oil of the row material: 
     (1) a distillate obtained by atmospheric distillation of paraffin base crude oils and/or mixed base crude oils; 
     (2) a distillate obtained by vacuum distillation of residual oils of paraffin base crude oils and/or mixed base crude oils (WVGO); 
     (3) a wax obtained through a lubricant oil dewaxing process (slack wax etc.) and/or synthetic wax obtained through a gas to liquid (GTL) process or the like (Fischer-Tropsch wax, GTL wax, etc.); 
     (4) a mixed oil of at least one selected from the base oils (1) to (3) and/or a mild hydrocracked oil of the mixed oil; 
     (5) a mixed oil of at least two selected from the base oils (1) to (4); (6) a deasphalted oil of the base oil (1), (2), (3), (4) or (5) (DAO); 
     (7) a mild hydrocracked oil of the base oil (6) (MHC); and 
     (8) a mixed oil of at least two selected from the base oils (1) to (7). 
     Preferred examples of the above described predetermined refining method include hydrorefining such as hydrocracking and hydrofinishing; solvent refining such as furfural solvent extraction; dewaxing such as solvent dewaxing and catalytic dewaxing; clay refining by acid clay, activated clay, etc.; and chemical (acid or alkali) washing such as sulfuric acid washing and caustic soda washing. In the present invention, these refining methods may be carried out individually, or at least two refining methods may be carried out in combination. When at least two refining methods are combined, the order thereof is not restricted, and can be suitably determined. 
     The following base oil (9) or (10) is especially preferable as the lubricant base oil according to this embodiment. The base oils (9) and (10) are obtained by carrying out a predetermined process on a base oil selected from the base oils (1) to (8), or on lubricant oil fractions recovered from any of the base oils (1) to (8): 
     (9) a hydrocracked base oil obtained by: hydrocracking a base oil selected from the base oils (1) to (8), or lubricant oil fractions recovered from any of the base oils (1) to (8); carrying out a dewaxing process such as solvent dewaxing and catalytic dewaxing on the products thereof, or lubricant oil fractions recovered from the products thereof by distillation or the like; and optionally further distilling the products thereof after the dewaxing process; and 
     (10) a hydroisomerized base oil obtained by: hydroisomerizing a base oil selected from the base oils (1) to (8), or lubricant oil fractions recovered from any of the base oils (1) to (8); carrying out a dewaxing process such as solvent dewaxing and catalytic dewaxing on the products thereof, or lubricant oil fractions recovered from the products thereof by distillation or the like; and optionally further distilling the products thereof after the dewaxing process. A catalytic dewaxing process is preferable as the dewaxing process. 
     When obtaining the above described lubricant base oil (9) or (10), a solvent refining process and/or hydrofinishing process may be further carried out at a suitable stage if necessary. 
     A catalyst used for the above described hydrocracking or hydroisomerization is not restricted, but a hydrocracking catalyst comprising a metal having hydrogenating ability (such as at least one metal in the VIa group and VIII group of the periodic table) supported on a composite oxide having cracking activity (for example, silica-alumina, alumina-boria and silica zirconia), or on at least one of the composite oxides in combination, bound by binders, as a support; or a hydroisomerization catalyst comprising at least one metal having hydrogenating ability including at least one group VIII metal supported on a support including zeolite (such as ZSM-5, zeolite beta, and SAPO-11) is preferably used. A hydrocracking catalyst and hydroisomerization catalyst may be used in combination by laminating or mixing or the like. 
     Reaction conditions of hydrocracking and hydroisomerization are not restricted. Preferably, the hydrogen partial pressure is 0.1 to 20 MPa, the average reaction temperature is 150 to 450° C., LHSV is 0.1 to 3.0 hr −1 , and the hydrogen/oil ratio is 50 to 20000 scf/b. 
     The kinematic viscosity of the lubricant base oil according to this embodiment at 100° C. is 2.0 to 5.0 mm 2 /s, preferably no more than 4.5 mm 2 /s, more preferably no more than 4.4 mm 2 /s, and especially preferably no more than 4.3 mm 2 /s; and preferably no less than 3.0 mm 2 /s, more preferably no less than 3.5 mm 2 /s, further preferably no less than 3.8 mm 2 /s, and especially preferably no less than 4.0 mm 2 /s. If the kinematic viscosity of the lubricant base oil at 100° C. is more than 5.0 mm 2 /s, low-temperature viscosity properties of the lubricating oil composition may deteriorate, and the fuel efficiency might be insufficient. If the kinematic viscosity thereof is less than 2.0 mm 2 /s, oil film formation at lubricating points might be insufficient, which causes poor lubricity, and evaporation loss of the lubricating oil composition might be large. 
     The kinematic viscosity of the lubricant base oil according to this embodiment at 40° C. is preferably no more than 40 mm 2 /s, more preferably no more than 30 mm 2 /s, further preferably no more than 25 mm 2 /s, especially preferably no more than 22 mm 2 /s, and most preferably no more than 20 mm 2 /s. On the other hand, the kinematic viscosity thereof at 40° C. is preferably no less than 10 mm 2 /s, more preferably no less than 14 mm 2 /s, further preferably no less than 16 mm 2 /s, especially preferably no less than 18 mm 2 /s, and most preferably no less than 19 mm 2 /s. If the kinematic viscosity of the lubricant base oil at 40° C. is more than 40 mm 2 /s, low-temperature viscosity properties of the lubricating oil composition may deteriorate, and the fuel efficiency might be insufficient. If the kinematic viscosity thereof is less than 10 mm 2 /s, oil film formation at lubricating points might be insufficient, which causes poor lubricity, and evaporation loss of the lubricating oil composition might be large. 
     In this specification, “kinematic viscosity at 40° C.” means kinematic viscosity at 40° C. specified by ASTM D-445. 
     The viscosity index of the lubricant base oil according to this embodiment is preferably no less than 100, more preferably no less than 110, further preferably no less than 120, especially preferably no less than 125, and most preferably no less than 130. If the viscosity index thereof is less than 100, not only viscosity-temperature characteristics, thermal and oxidation stability and anti-evaporation performance deteriorate, but also the friction coefficient tends to increase and anti-wear properties tends to decrease. The viscosity index in this specification means a viscosity index measured conforming to JIS K 2283-1993. 
     The density of the lubricant base oil according to this embodiment at 15° C. (   15 ) is preferably no more than 0.860, more preferably no more than 0.850, further preferably no more than 0.840, and especially preferably no more than 0.835. The density at 15° C. in this specification means density measured at 15° C., conforming to JIS K 2249-1995. 
     The pour point of the lubricant base oil according to this embodiment is preferably no more than −10° C., more preferably no more than −12.5° C., further preferably no more than −15° C., especially preferably no more than −17.5° C., and most preferably no more than −20.0° C. If the pour point is beyond the above described upper limit, low-temperature fluidity of whole of the lubricating oil composition tends to deteriorate. The pour point in this specification means a pout point measured conforming to JIS K 2269-1987. 
     The sulfur content in the lubricant base oil according to this embodiment depends on the sulfur content in its raw material. For example, in a case where a substantially sulfur-free raw material such as a synthetic wax component obtained through Fischer-Tropsch reaction or the like, is used, a substantially sulfur-free lubricant base oil can be obtained. In a case where a raw material containing sulfur, such as a slack wax obtained through the process of refining the lubricant base oil, and a microwax obtained through a wax refining process, is used, the sulfur content in the obtained lubricant base oil is usually no less than 100 mass ppm. In the lubricant base oil according to this embodiment, in view of further improvement of the thermal and oxidation stability and reduction of the sulfur content, the sulfur content is preferably no more than 100 mass ppm, more preferably no more than 50 mass ppm, further preferably no more than 10 mass ppm, and most preferably no more than 5 mass ppm. 
     The nitrogen content in the lubricant base oil according to this embodiment is preferably no more than 10 mass ppm, more preferably no more than 5 mass ppm, and further preferably no more than 3 mass ppm. If the nitrogen content is beyond 10 mass ppm, the thermal and oxidation stability tends to deteriorate. The nitrogen content in this specification means nitrogen content measured conforming to JIS K 2609-1990. 
     Preferably, % C P  of the lubricant base oil according to this embodiment is no less than 70, more preferably no less than 80, and further preferably no less than 85; and usually no more than 99, preferably no more than 95, and more preferably no more than 94. In a case where % C P  of the lubricant base oil is under the above lower limit, the viscosity-temperature characteristics, thermal and oxidation stability and friction properties tend to deteriorate, and effects of an additive tend to decrease when the additive is incorporated to the lubricant base oil. In a case where % C P  of the lubricant base oil is beyond the above upper limit, solubility of an additive tends to decrease. 
     Preferably, % C A  of the lubricant base oil according to this embodiment is no more than 2, more preferably no more than 1, further preferably no more than 0.8, and especially preferably no more than 0.5. In a case where % C A  of the lubricant base oil is beyond the above upper limit, the viscosity-temperature characteristics, thermal and oxidation stability and fuel efficiency tend to deteriorate. 
     Preferably, % C N  of the lubricant base oil according to this embodiment is no more than 30, more preferably no more than 25, further preferably no more than 20, and especially preferably no more than 15. Preferably, % C N  of the lubricant base oil is no less than 1, and more preferably no less than 4. In a case where % C N  of the lubricant base oil is beyond the above upper limit, the viscosity-temperature characteristics, thermal and oxidation stability and friction properties tend to deteriorate. In a case where % C N  thereof is under the above lower limit, solubility of an additive tends to decrease. 
     In this specification, % C P , % C N  and % C A  mean percentage of the paraffin carbon number to all the carbon atoms, percentage of the naphthene carbon number to all the carbon atoms, and percentage of the aromatic carbon number to all the carbon atoms, respectively, obtained by the method conforming to ASTM D 3238-85 (n-d-M ring analysis). That is, the above described preferred ranges of % C P , % C N , and % C A  are based on values obtained according to the above method. For example, the value of % C N  obtained according to the above method can indicate more than 0 even if the lubricant base oil does not contain naphthenes. 
     The saturated content in the lubricant base oil according to this embodiment is preferably no less than 90 mass %, more preferably no less than 95 mass %, and further preferably no less than 99 mass %, on the basis of the total mass of the lubricant base oil. The proportion of the cyclic-saturated content to the saturated content is preferably no more than 40 mass %, preferably no more than 35 mass %, preferably no more than 30 mass %, more preferably no more than 25 mass %, and further preferably no more than 21 mass %. The proportion of the cyclic saturated content to the saturated content is also preferably no less than 5 mass %, and more preferably no less than 10 mass %. The saturated content and the proportion of the cyclic-saturated content to the saturated content within the above range makes it possible to improve the viscosity-temperature characteristics, and thermal and oxidation stability. In a case where an additive is incorporated to the lubricant base oil, functions of the additive can be brought out at a higher level while the additive is sufficiently stably dissolved and retained in the lubricant base oil. Further, friction properties of the lubricant base oil itself can be improved, and as a result, friction-reducing performance can be improved, which leads to improvement of the energy efficiency. In this specification, the saturated content means a value measured conforming to ASTM D 2007-93. 
     Any of similar methods from which the same results are obtained can be used for each of a method of separating the saturated content, and the composition analysis of e.g. the cyclic saturated content, and the noncyclic saturated content. Examples thereof include the above method specified in ASTM D 2007-93, the method specified in ASTM D 2425-93, the method specified in ASTM D 2549-91, methods using high performance liquid chromatography (HPLC), and methods obtained by improving these methods. 
     The aromatic content in the lubricant base oil according to this embodiment is preferably no more than 10 mass %, more preferably no more than 5 mass %, further preferably no more than 4 mass %, especially preferably no more than 3 mass %, and most preferably no more than 2 mass %, on the basis of the total mass of the lubricant base oil; may be 0 mass %; and is preferably no less than 0.1 mass %, more preferably no less than 0.5 mass %, further preferably no less than 1 mass %, and especially preferably no less than 1.5 mass %. In a case where the aromatic content is beyond the above upper limit, the viscosity-temperature characteristics, thermal and oxidation stability, friction properties, and further, anti-evaporation performance and the low-temperature viscosity properties tend to deteriorate. Further, in a case where an additive is incorporated to the lubricant base oil, effects of the additive tend to decrease. Although the lubricant base oil according to this embodiment does not have to contain the aromatic content, the aromatic content no less than the above described lower limit makes it possible to further improve solubility of an additive. 
     In this specification, the aromatic content means a value measured conforming to ASTM D 2007-93. The aromatic content usually includes alkylbenzenes, and alkylnaphthalenes; anthracenes, phenanthrenes, and alkylated compounds thereof; and compounds having four or more fused benzene rings, aromatic compounds having hetero atoms such as pyridines, quinolines, phenols, and naphthols. 
     A synthetic base oil may be used as the lubricant base oil according to this embodiment. Examples of the synthetic base oil include poly-α-olefins, and hydrogenated products thereof; isobutene oligomers, and hydrogenated products thereof; isoparaffins; alkylbenzenes; alkylnaphthalenes; diesters (such as ditridecyl glutarate, bis(2-ethylhexyl) azipate, diisodecyl azipate, ditridecyl azipate, and bis(2-ethylhexyl) sebacate); polyol esters (such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate, and pentaerythritol pelargonate); polyoxyalkylene glycols; dialkyl diphenyl ethers; polyphenyl ethers; and mixtures thereof, having a kinematic viscosity of 2.0 to 5.0 mm 2 /s at 100° C. Among them, poly-α-olefins are preferable. Examples of poly-α-olefins typically include oligomers and co-oligomers of C 2 -C 32 , preferably C 6 -C 16  α-olefins (such as 1-octene oligomers, decene oligomers, and ethylene-propylene co-oligomers) and hydrogenated products thereof. 
     The method for producing a poly-α-olefin is not restricted. Examples thereof include a method of polymerizing an α-olefin in the presence of a polymerization catalyst such as a catalyst containing a complex of aluminum trichloride or boron trifluoride, and water, alcohol (such as ethanol, propanol, and butanol), a carboxylic acid or an ester. 
     The lubricant base oil according to this embodiment either may be composed of one base oil component, or may contain a plurality of base oil components, as long as the base oil as a whole has a kinematic viscosity at 100° C. of 2.0 to 5.0 mm 2 /s. 
     &lt;(B) Metallic Detergent&gt; 
     The lubricant base oil of the present invention contains (B1) a calcium-containing metallic detergent (hereinafter may be referred to as “component (B1)”) and (B2) a magnesium-containing metallic detergent (hereinafter may be referred to as “component (B2)”) as (B) a metallic detergent (hereinafter may be referred to as “component (B)”). Examples of the component (B) include phenate detergents, sulfonate detergents and salicylate detergents. These metallic detergents can be used alone or in combination. 
     Preferred examples of a phenate detergent include overbased salts of alkaline earth metal salts of compounds having the structure of the following formula (1). Examples of alkaline earth metals include magnesium, barium, and calcium. Among them, magnesium and calcium are preferable. 
     
       
         
         
             
             
         
       
     
     In the formula (1), R 1  is a C 6 -C 21  linear or branched, saturated or unsaturated alkyl or alkenyl group; m is a polymerization degree, representing an integer of 1 to 10; A is a sulfide (—S—) group or methylene (—CH 2 —) group; and x is an integer of 1 to 3. R 1  may be combination of at least two different groups. 
     The carbon number of R 1  in the formula (1) is preferably 9 to 18, and more preferably 9 to 15. If the carbon number of R 1  is less than 6, the solubility in the base oil might be poor. On the other hand, if the carbon number of R 1  is beyond 21, it is difficult to produce the compound and the thermal stability might be poor. 
     The polymerization degree m in the formula (1) is preferably 1 to 4. The polymerization degree m within this range makes it possible to improve the thermal stability. 
     Preferred examples of a sulfonate detergent include alkaline earth metal salts of alkyl aromatic sulfonic acids obtained by sulfonation of alkylaromatics, or basic or overbased salts thereof. The weight-average molecular weight of the alkylaromatics is preferably 400 to 1500, and more preferably 700 to 1300. 
     Examples of alkaline earth metals include magnesium, barium, and calcium, and magnesium and calcium are preferable. Examples of alkyl aromatic sulfonic acids include what is called petroleum sulfonic acids and synthetic sulfonic acids. Examples of petroleum sulfonic acids here include sulfonated products of alkylaromatics of lubricant oil fractions derived from mineral oils, and what is called mahogany acid, which is a side product of production of white oils. Examples of synthetic sulfonic acids includes a sulfonated product of alkylbenzene having a linear or branched alkyl group, obtained by recovering side products in a manufacturing plant of alkylbenzene, which is a raw material of detergents, or by alkylating benzene with polyolefins. Other examples of synthetic sulfonic acids includes a sulfonated product of alkylnaphthalenes such as dinonylnaphthalene. A sulfonating agent used when sulfonating these alkylaromatics is not limited. For example, a fuming sulfuric acid or a sulfuric anhydride can be used as the sulfonating agent. 
     Preferred examples of a salicylate detergent include metallic salicylates or basic or overbased salts thereof. Preferred examples of metallic salicylates here include compounds represented by the following formula (2): 
     
       
         
         
             
             
         
       
     
     In the above formula (2), each R 2  is independently a C 14 -C 30  alkyl or alkenyl group; M is an alkaline earth metal; and n is 1 or 2. M is preferably calcium or magnesium. Preferably n is 1. When n=2, R 2  may be combination of different groups. 
     A preferred embodiment of a salicylate detergent can be an alkaline earth metal salicylate of the above formula (2) wherein n=1, or a basic or overbased salt thereof. 
     A method for producing alkaline earth metal salicylate is not restricted, and known methods for producing monoalkylsalicylates can be used. For example, an alkaline earth metal salicylate can be obtained by: reacting a metal base such as an oxide and hydroxide of an alkaline earth metal with a monoalkylsalicylic acid obtained by alkylating a phenol as a starting material with an olefin, and then carboxylating the resultant with a carbonic acid gas or the like, or a monoalkylsalicylic acid obtained by alkylating a salicylic acid as a starting material with an equivalent of the olefin, or the like; once converting the above monoalkylsalicylic acid or the like to an alkali metal salt such as a sodium salt and potassium salt, and then performing transmetallation with an alkaline earth metal salt; or the like. 
     The metallic detergent may be overbased by a carbonate salt (for example, an alkaline earth metal carbonate salt such as calcium carbonate and magnesium carbonate), or a borate salt (for example, an alkaline earth metal borate salt such as calcium borate and magnesium borate). 
     A method for obtaining an alkaline earth metal carbonate salt-overbased metallic detergent is not limited. For example, such a metallic detergent can be obtained by reacting a neutral salt of the metallic detergent (such as an alkaline earth metal phenate, an alkaline earth metal sulfonate, and an alkaline earth metal salicylate) with a base of an alkaline earth metal (such as a hydroxide and an oxide of an alkaline earth metal) in the presence of carbonic acid gas. 
     A method for obtaining an alkaline earth metal borate salt-overbased metallic detergent is not limited. Such a metallic detergent can be obtained by reacting a neutral salt of a metallic detergent (such as an alkaline earth metal phenate, an alkaline earth metal sulfonate, and an alkaline earth metal salicylate) with a base of an alkaline earth metal (such as a hydroxide and an oxide of an alkaline earth metal) in the presence of a boric acid or a boric acid anhydride, or a borate salt. 
     Examples of the component (B1) include calcium phenate detergents, calcium sulfonate detergents, calcium salicylate detergents, and combination thereof. Preferably, the component (B1) contains at least an overbased calcium salicylate detergent. The component (B1) may be either calcium carbonate-overbased, or calcium borate-overbased. 
     Examples of the component (B2) include magnesium phenate detergents, magnesium sulfonate detergents, magnesium salicylate detergents, and combination thereof. Preferably, the component (B2) contains an overbased magnesium sulfonate detergent. The component (B2) may be either magnesium carbonate-overbased, or magnesium borate-overbased. 
     The metal ratio of the component (B) is a value calculated according to the following formula; and is preferably no less than 1.0, and more preferably no less than 1.5; and preferably no more than 10, and more preferably no more than 3.0. 
     The metal ratio of the component (B)=the valence of the metal element in the component (B)×the metal content in the component (B) (mol)/the soap group content of the component (B) (mol) 
     In a case where a boric acid salt-overbased alkaline earth metal salicylate is contained as the component (C) described later, the metal ratio of this boric acid salt-overbased alkaline earth metal salicylate is preferably no less than 1.0, and more preferably no less than 1.5; and preferably no more than 3.0, more preferably no more than 2.5, and further preferably no more than 2.0. 
     The content of the component (B) in the lubricating oil composition is, in terms of calcium on the basis of the total mass of the lubricating oil composition, 500 to 2500 mass ppm, preferably no less than 1000 mass ppm, and more preferably no less than 1200 mass ppm; and preferably no more than 2000 mass ppm, and more preferably no more than 1600 mass ppm. If the content in terms of calcium is beyond 2500 mass ppm, LSPI is easy to occur. The content in terms of calcium no less than the above described lower limit makes it possible to maintain high detergency inside an engine, and to improve base number retention. 
     The content of the component (B) in the lubricating oil composition is, in terms of magnesium on the basis of the total mass of the lubricating oil composition, 100 to 1000 mass ppm, preferably no less than 150 mass ppm, and more preferably no less than 200 mass ppm; and preferably no more than 800 mass ppm, and more preferably no more than 500 mass ppm. The content in terms of magnesium no less than the above described lower limit makes it possible to improve detergency of an engine while suppressing LSPI. The content in terms of magnesium no more than the above described upper limit makes it possible to suppress increase of friction coefficients. 
     &lt;(C) Boron-Containing Additive&gt; 
     The lubricating oil composition of the present invention contains (C) a boron-containing additive that is oil-soluble or oil-dispersible and is stable in oil (hereinafter may be simply referred to as “component (C)”). The component (C) may compose at least part of the component (B). 
     (C1) a boric acid salt-overbased metallic detergent (hereinafter may be simply referred to as “component (C1)”), and/or (C2) a boronated ashless dispersant (hereinafter may be simply referred to as “component (C2)”) can be preferably used as the component (C). Preferably the component (C) contains at least the component (C1). 
     In a case where the lubricating oil composition contains the component (C1), the component (C1) composes at least part of the component (B). Examples of the component (C1) include a boric acid salt-overbased alkaline earth metal phenate, a boric acid salt-overbased alkaline earth metal salicylate, and a boric acid salt-overbased alkaline earth metal sulfonate. Among them, a boric acid salt-overbased alkaline earth metal salicylate can be preferably used. The component (C1) may compose at least part of the component (B1), or may compose at least part of the component (B2), or may compose at least part of the component (B1) and part of the component (B2), or may compose neither the component (B1) nor (B2). 
     Preferred examples of the component (C2) include boronated products of nitrogen-containing ashless dispersants. Examples of nitrogen-containing ashless dispersants to be boronated include at least one nitrogen-containing ashless dispersant selected from the following (C2a′) to (C2c′): 
     (C2a′) succinimide having at least one alkyl or alkenyl group in its molecule (hereinafter may be referred to as “ashless dispersant (C2a′)”); 
     (C2b′) benzylamine having at least one alkyl or alkenyl group in its molecule (hereinafter may be referred to as “ashless dispersant (C2b′)”); and 
     (C2c′) polyamine having at least one alkyl or alkenyl group in its molecule (hereinafter may be referred to as “ashless dispersant (C2c′)”). 
     Hereinafter a boronated product of the ashless dispersant (C2a′) may be referred to as “component (C2a)”, a boronated product of the ashless dispersant (C2b′) may be referred to as “component (C2b)”, and a boronated product of the ashless dispersant (C2c′) may be referred to as “component (C2c)”. 
     Among them, the component (C2a) can be especially preferably used. 
     Examples of the ashless dispersant (C2a′) include compounds represented by the following formula (3) or (4): 
     
       
         
         
             
             
         
       
     
     In the formula (3), R 3  is a C 40 -C 400  alkyl or alkenyl group; h represents an integer of 1 to 5, preferably 2 to 4. The carbon number of R 3  is preferably no less than 60, and preferably no more than 350. 
     In the formula (4), R 4  and R 5  are independently C 40 -C 400  alkyl or alkenyl group, and may be combination of different groups. R 4  and R 5  are especially preferably polybutenyl groups. In addition, i represents an integer of 0 to 4, preferably 1 to 3. The carbon number of R 4  and R 5  is preferably no less than 60, and preferably no more than 350. 
     The above lower limits or over of the carbon numbers of R 3  to R 5  in the formulas (3) and (4) make it possible to obtain good solubility in the lubricant base oil. On the other hand, the above upper limits or below of the carbon numbers of R 3  to R 5  make it possible to improve low-temperature fluidity of the lubricating oil composition. 
     The alkyl or alkenyl groups (R 3  to R 5 ) in formulae (3) and (4) may be linear or branched. Preferred examples thereof include branched alkyl groups and branched alkenyl groups derived from oligomers of olefins such as propene, 1-butene, and isobutene, or from co-oligomers of ethylene and propylene. Among them, a branched alkyl or alkenyl group derived from oligomers of isobutene that are conventionally referred to as polyisobutylene, or a polybutenyl group are most preferable. 
     Preferred number-average molecular weight of the alkyl or alkenyl groups (R 3  to R 5 ) in formulae (3) and (4) is 800 to 3500. 
     Succinimide having at least one alkyl or alkenyl group in its molecule includes so-called monotype succinimide represented by the formula (3), where a succinic anhydride terminates only one end of a polyamine chain, and so-called bistype succinimide represented by the formula (4), where succinic anhydrides terminate both ends of a polyamine chain. The lubricating oil composition of the present invention may include either monotype or bistype succinimide, and may include both of them as a mixture. 
     A method for producing a succinimide having at least one alkyl or alkenyl group in its molecule is not limited. For example, such succinimide can be obtained by: reacting an alkyl succinic acid or an alkenyl succinic acid obtained by reacting a compound having a C 40 -C 400  alkyl or alkenyl group with maleic anhydride at 100 to 200° C., with a polyamine. Here, examples of polyamine include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine. 
     Examples of the ashless dispersant (C2b′) include compounds represented by the following formula (5): 
     
       
         
         
             
             
         
       
     
     In the formula (5), R 6  is a C 40 -C 400  alkyl or alkenyl group; and j represents an integer of 1 to 5, preferably 2 to 4. The carbon number of R 6  is preferably no less than 60, and preferably no more than 350. 
     A method for producing the ashless dispersant (C2b′) is not limited. An example of such a method include: reacting a polyolefin such as propylene oligomer, polybutene, and ethylene-α-olefin copolymer, with phenol, to give an alkylphenol; and then reacting the alkylphenol with formaldehyde, and a polyamine such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine, by Mannich reaction. 
     Examples of the ashless dispersant (C2c′) include compounds represented by the following formula (6): 
       [Chem. 5] 
       R 7 —NH—(CH 2 CH 2 NH) k —H  (6)
 
     In the formula (6), R 7  is a C 40 -C 400  alkyl or alkenyl group; k represents an integer of 1 to 5, preferably 2 to 4. The carbon number of R 7  is preferably no less than 60, and preferably no more than 350. 
     A method for producing the ashless dispersant (C2c′) is not limited. An example of such a method include: chlorinating a polyolefin such as propylene oligomer, polybutene, and ethylene-α-olefin copolymer; and then reacting the chlorinated polyolefin with ammonia, or a polyamine such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine. 
     The components (C2a) to (C2c), that is, the boronated products of the ashless dispersants (C2a′) to (C2c′) can be obtained by, for example, reacting the ashless dispersants (C2a′) to (C2c′) with boric acid, and neutralizing or amidating part or all of the residual amino groups and/or imino groups with boric acid. Boronation may be performed in combination with modification by other reagents described below. 
     The content of the component (C) in the lubricating oil composition is, in terms of boron on the basis of the total mass of the lubricating oil composition, 50 to 1000 mass ppm, preferably no less than 190 mass ppm, more preferably no less than 270 mass ppm, and especially preferably no less than 400 mass ppm; and preferably no more than 800 mass ppm. The boron content derived from the component (C) of the above lower limit or over makes it possible to improve fuel efficiency. The boron content derived from the component (C) of the above upper limit or below makes it possible to maintain fuel efficiency. 
     In one preferred embodiment, the lubricating oil composition contains at least the component (C1) as the component (C), and more preferably at least a boric acid salt-overbased alkaline earth metal salicylate as the component (C). The alkaline earth metal of the boric acid salt-overbased alkaline earth metal salicylate is preferably calcium and/or magnesium. 
     In a case where the lubricating oil composition contains at least the component (C1) as the component (C), the content of the component (C1) is, in terms of boron on the basis of the total mass of the lubricating oil composition, preferably no less than 200 mass ppm, more preferably no less than 300 mass ppm, and especially preferably no less than 400 mass ppm; and preferably no more than 700 mass ppm. The boron content derived from the component (C1) within the above range makes it easy to improve fuel efficiency. 
     In another preferred embodiment, the lubricating oil composition contains the components (C1) and (C2) as the component (C). In a case where the lubricating oil composition contains the components (C1) and (C2) as the component (C), the content of the component (C2) is, in terms of boron on the basis of the total mass of the lubricating oil composition, preferably no less than 50 mass ppm, and more preferably no less than 100 mass ppm; and preferably no more than 400 mass ppm. The boron content derived from the component (C2) within the above range makes it easy to improve fuel efficiency. 
     &lt;(D) Oil-Soluble Organic Molybdenum Compound&gt; 
     The lubricating oil composition of the present invention contains (D) an oil-soluble organic molybdenum compound (hereinafter may be referred to as “component (D)”) in an amount of 100 to 2000 mass ppm in terms of molybdenum on the basis of the total mass of the lubricating oil composition. As the component (D), preferably (D1) a molybdenum dithiocarbamate (sulfurized molybdenum dithiocarbamate or sulfurized oxymolybdenum dithiocarbamate. Hereinafter this may be referred to as “component (D1)”) is contained. 
     For example, a compound represented by the following formula (7) can be used as the component (D1): 
     
       
         
         
             
             
         
       
     
     In the above general formula (7), R 8  to R 11  may be either the same or different, and is a C 2 -C 24  alkyl or C 6 -C 24  (alkyl)aryl group, preferably a C 4 -C 13  alkyl or C 10 -C 15  (alkyl)aryl group. This alkyl group may be a primary, secondary, or tertiary alkyl group, and may be linear or branched. It is noted that “(alkyl)aryl group” means “aryl or alkylaryl group”. In an alkylaryl group, the alkyl substituent may be in any position of the aromatic ring. Y 1  to Y 4  are each independently a sulfur atom or oxygen atom. At least one of Y 1  to Y 4  is a sulfur atom. 
     Examples of the oil-soluble organic molybdenum compound other than the component (D1) include molybdenum dithiophosphate; complexes or the like of a molybdenum compound (e.g. molybdenum oxides such as molybdenum dioxide and molybdenum trioxide; molybdic acids such as orthomolybdic acid, paramolybdic acid, and sulfurized (poly)molybdic acid; molybdate salts such as metal salts and ammonium salts of these molybdic acids; molybdenum sulfides such as molybdenum disulfide, molybdenum trisulfide, molybdenum pentasulfide, and molybdenum polysulfide; sulfurized molybdic acid, and metal salts or amine salts of thereof; and molybdenum halides such as molybdenum chloride) and a sulfur-containing organic compound (such as alkyl (thio)xanthate, thiadiazole, mercaptothiadiazole, thiocarbonate, tetrahydrocarbyl thiuram disulfide, bis(di(thio)hydrocarbyl dithiophosphonate) disulfide, organic (poly)sulfide and sulfurized ester) or other organic compound; and sulfur-containing organic molybdenum compounds such as complexes of a sulfur-containing molybdenum compound (such as the above described molybdenum sulfides, and sulfurized molybdic acid), and an alkenylsuccinimide. These organic molybdenum compounds may be either mononuclear molybdenum compounds, or polynuclear molybdenum compounds such as binuclear or trinuclear molybdenum compounds. 
     As the oil-soluble organic molybdenum compound other than the component (D1), an organic molybdenum compound that does not contain sulfur as a constituent element can be used. Specific examples of such an organic molybdenum compound that does not contain sulfur as a constituent element include: molybdenum-amine complexes, molybdenum-succinimide complexes, molybdenum salts of organic acids, molybdenum salts of alcohols, or the like. Among them, molybdenum-amine complexes, molybdenum salts of organic acids, or molybdenum salts of alcohols are preferable. 
     The content of the component (D) in the lubricating oil composition is, in terms of molybdenum on the basis of the total mass of the lubricating oil composition, 100 to 2000 mass ppm, preferably no less than 500 mass ppm, more preferably no less than 700 mass ppm, and especially preferably no less than 900 mass ppm; and preferably no more than 1500 mass ppm. In a case where the content of the component (D) is less than the above lower limit, the effect of reducing friction by addition thereof tends to be insufficient, and fuel efficiency and thermal and oxidation stability of the lubricating oil composition tend to be insufficient. In a case where the content of the component (D) is more than the above upper limit, effect commensurate to the content is not obtained, and storage stability of the lubricating oil composition tends to deteriorate. 
     In a case where the component (D) contains the component (D1), the content of the component (D1) is, in terms of molybdenum on the basis of the total mass of the lubricating oil composition, preferably no less than 300 mass ppm, more preferably no less than 500 mass ppm, further preferably no less than 600 mass ppm, and especially preferably no less than 700 mass ppm; and preferably no more than 1200 mass ppm, and more preferably no more than 1000 mass ppm. The molybdenum content of the above lower limit or over makes it possible to improve fuel efficiency and LSPI suppression performance. The molybdenum content of the above upper limit or below makes it possible to improve storage stability of the lubricating oil composition. 
     &lt;Ashless Dispersant&gt; 
     The lubricating oil composition of the present invention may contain an ashless dispersant which falls under the above component (C) (that is, the above component (C2)), or may contain an ashless dispersant which does not fall under the above component (C), or may contain both thereof. Examples of the ashless dispersant which does not fall under the above component (C) include the above described ashless dispersants (C2a′) to (C2c′), and derivatives thereof other than the boronated products of the ashless dispersants (C2a′) to (C2c′). 
     Examples of derivatives of the ashless dispersants (C2a′) to (C2c′) other than the boronated products include: 
     (i) an oxygen-containing organic compound-modified product where a part or all of the residual amino and/or imino groups is/are neutralized or amidated by reacting the ashless dispersants (C2a′) to (C2c′) with a C 1 -C 30  monocarboxylic acid such as fatty acids, a C 2 -C 30  polycarboxylic acid (such as ethanedioic acid, phthalic acid, trimellitic acid, and pyromellitic acid), an anhydride or ester thereof, a C 2 -C 6  alkylene oxide, or a hydroxy(poly)oxyalkylene carbonate; 
     (ii) a phosphoric acid-modified product where a part or all of the residual amino and/or imino groups is/are neutralized or amidated by reacting the ashless dispersants (C2a′) to (C2c′) with phosphoric acid; and 
     (iii) a sulfur-modified product obtained by reacting the ashless dispersants (C2a′) to (C2c′) with a sulfur compound. 
     Modification of these (i) to (iii) may be carried out in combination. 
     The molecular weight of the ashless dispersant is not restricted. Preferred weight-average molecular weight thereof is 1000 to 20000. 
     In a case where the lubricating oil composition contains the ashless dispersant, the total content of all the ashless dispersant contained in the lubricating oil composition is, in terms of nitrogen on the basis of the total mass of the lubricating oil composition, preferably no less than 100 mass ppm, more preferably no less than 300 mass ppm, and further preferably no less than 400 mass ppm; and preferably no more than 2000 mass ppm, and more preferably no more than 1000 mass ppm, irrespective of whether or not each ashless dispersant contains boron (that is, whether or not each ashless dispersant contributes to the content of the component (C)). The content of all the ashless dispersant of the above lower limit or over makes it possible to sufficiently improve anti-coking performance (thermal stability) of the lubricating oil composition. The content of all the ashless dispersant of the above upper limit or below makes it possible to maintain high fuel efficiency. 
     &lt;Other Additives&gt; 
     Other additives that are generally used in lubricating oils can be contained in the lubricating oil composition of the present invention according to its purpose in order to further improve its performance. Examples of such additives include: zinc dialkyldithiophosphate, antioxidants, ashless friction modifiers, anti-wear additives or extreme pressure agents, viscosity index improvers or pour point depressants, corrosion inhibitors, anti-rust agents, metal deactivators, demulsifiers, and anti-foaming agents. 
     As zinc dialkyldithiophosphate, for example, a compound represented by the following formula (8) can be used: 
     
       
         
         
             
             
         
       
     
     In the formula (8), R 12  to R 15  are independently a C 1 -C 24  linear or branched alkyl group, and may be combination of different groups. The carbon numbers of R 12  to R 15  are preferably no less than 3, preferably no more than 12, and more preferably no more than 8. R 12  to R 15  may be primary, secondary, or tertiary alkyl groups, preferably primary or secondary alkyl groups, or combination thereof. Preferably, the mole ratio of primary alkyl group and secondary alkyl group (primary alkyl group:secondary alkyl group) is 0:100 to 30:70. This ratio may be the intramolecular combination ratio of alkyl chains, or may be the mixing ratio of ZnDTP having only primary alkyl groups and ZnDTP having only secondary alkyl groups. When secondary alkyl groups are major, fuel efficiency can be improved. 
     A method for producing the above zinc dialkyldithiophosphate is not limited. For example, zinc dialkyldithiophosphate can be synthesized by: reacting alcohol(s) having an alkyl group corresponding to R 12  to R 15  with phosphorus pentasulfide, to synthesize dithiophosphoric acid; and neutralizing the dithiophosphoric acid with zinc oxide. 
     In a case where the lubricating oil composition contains ZnDTP, the content thereof is, in terms of phosphorus on the basis of the total mass of the composition, preferably no less than 600 mass ppm, more preferably no less than 700 mass ppm, and especially preferably no less than 800 mass ppm; and preferably no more than 1000 mass ppm. The ZnDTP content of the above lower limit or over makes it possible to improve not only oxidation stability but also LSPI suppression performance. The ZnDTP content beyond the above upper limit brings about unfavorable significant poisoning of an exhaust gas treatment catalyst. 
     Known antioxidants such as phenolic antioxidants and amine antioxidants can be used as the antioxidant. Examples thereof include: amine antioxidants such as alkylated diphenylamine, phenyl-α-naphthylamine, and alkylated α-naphthylamine; and phenolic antioxidants such as 2,6-di-t-butyl-4-methylphenol, and 4,4′-methylenebis(2,6-di-t-butylphenol). 
     In a case where the lubricating oil composition contains an antioxidant, the content thereof is usually no more than 5.0 mass %, preferably no more than 3.0 mass %; and preferably no less than 0.1 mass %, more preferably no less than 0.5 mass %, on the basis of the total mass of the lubricating oil composition. 
     As the ashless friction modifier, compounds usually used as friction modifiers for lubricant oils can be used without particular limitation. Examples of the ashless friction modifier include C 6 -C 50  compounds containing, in each of their molecules, at least one heteroatom selected from the group of an oxygen atom, a nitrogen atom and a sulfur atom. More specific examples thereof include ashless friction modifiers such as amines, fatty acid esters, fatty acid amides, fatty acids, aliphatic alcohols, aliphatic ethers, urea compounds, and hydrazide compounds, each having at least one C 6 -C 30  alkyl or alkenyl group, especially C 6 -C 30  linear alkyl, linear alkenyl, branched alkyl, or branched alkenyl group, in each of their molecules. 
     In a case where the lubricating oil composition contains an ashless friction modifier, the content thereof is preferably no less than 0.01 mass %, more preferably no less than 0.1 mass %, and further preferably no less than 0.3 mass %; and preferably no more than 2 mass %, more preferably no more than 1 mass %, and especially preferably no more than 0.8 mass % on the basis of the total mass of the lubricating oil composition. The content of the ashless friction modifier of less than 0.01 mass % tends to lead to insufficient friction reducing effect by addition thereof. The content thereof beyond 2 mass % tends to inhibit effects of anti-wear additives etc., or to deteriorate solubility of additives. 
     Anti-wear agents or extreme pressure agents used for lubricating oils can be used as the anti-wear agent or extreme pressure agent without particular limitation. Examples thereof include sulfur-based, phosphorous-based, and sulfur-phosphorous-based extreme pressure agents. Specific examples include phosphite esters, thiophosphite esters, dithiophosphite esters, trithiophosphite esters, phosphate esters, thiophosphate esters, dithiophosphate esters, trithiophosphate esters, amine salts thereof, metal salts thereof, derivatives thereof, dithiocarbamates, zinc dithiocarbamate, disulfides, polysulfides, sulfurized olefins, and sulfurized oils. Among them, addition of a sulfur-based extreme pressure agent, especially a sulfurized oil is preferable. In a case where the lubricating oil composition contains an anti-wear agent or extreme pressure agent, the content thereof is preferably 0.01 to 10 mass % based on the total mass of the lubricating oil composition. 
     Non-dispersant viscosity index improvers and dispersant viscosity index improvers can be used as the viscosity index improver. Specific examples thereof include: non-dispersant or dispersant polymethacrylates, olefin copolymers, polyisobutenes, polystyrenes, ethylene-propylene copolymers, styrene-diene copolymers and hydrogenated products thereof, or the like. Their weight-average molecular weights are generally 5,000 to 1,000,000. In order to further improve the fuel efficiency, it is desirable to use the above viscosity index improver having a weight average molecular weight of 100,000 to 1,000,000, preferably 200,000 to 900,000, especially preferably 400,000 to 800,000. In the lubricating oil composition of the present invention, in view of improvement of fuel efficiency, a poly(meth)acrylate viscosity index improver comprising 30 to 90 mole % of the structural units represented by the following general formula (9) and 0.1 to 50 mole % of the structural units represented by the following general formula (10), wherein the hydrocarbon main chain ratio is no more than 0.18, can be especially preferably used. In this specification, “hydrocarbon main chain ratio” of a poly(meth)acrylate viscosity index improver means the proportion of the carbon number derived from the main chain to the total carbon number of the poly(meth)acrylate viscosity index improver (the carbon number of the main chain/the total carbon number). 
     
       
         
         
             
             
         
       
     
     In the above general formula (9), R 16  is a hydrogen atom or a methyl group, and R 17  is a linear or branched hydrocarbon group having a carbon number of no more than 6. In the general formula (10), R 18  is a hydrogen atom or a methyl group, and R 19  is a linear or branched hydrocarbon group having a carbon number of no less than 16. 
     This viscosity index improver preferably has a PSSI (Permanent Shear Stability Index) in a Diesel Injector method of no more than 30. If PSSI is beyond 30, shear stability is poor, and keeping a certain level or better of a kinematic viscosity and a HTHS viscosity of an oil after use might sacrifice fuel efficiency at the early stage of use. 
     “PSSI in a Diesel Injector method” mentioned here means a permanent shear stability index of a polymer calculated based on data measured according to the method specified in ASTM D6278-02 (Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus), conforming to ASTM D6022-01 (Standard Practice for Calculation of Permanent Shear Stability Index). 
     When the lubricating oil composition contains a viscosity index improver, the content thereof is generally beyond 0 mass % and no more than 20 mass % on the basis of the total mass of the lubricating oil composition. Specific content thereof may be, for example, such content that the lubricating oil composition can have desirable viscosity properties (kinematic vescocity, viscosity index, and HTHS viscosity) described below. 
     Examples of the pour point depressant include polymethacrylate polymers. In a case where the lubricating oil composition contains a pour point depressant, the content thereof is usually 0.01 to 2 mass % on the basis of the total mass of the lubricating oil composition. 
     Known corrosion inhibitors such as benzotriazole compounds, tolyltriazole compounds, thiadiazole compounds, and imidazole compounds can be used as the corrosion inhibitor. In a case where the lubricating oil composition contains a corrosion inhibitor, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition. 
     Known anti-rust agents such as petroleum sulfonates, alkylbenzenesulfonates, dinonylnaphthalenesulfonates, alkylsulfonate salts, fatty acids, alkenylsuccinimide half esters, fatty acid soaps, fatty acid polyol esters, fatty acid amine salts, oxidized paraffins, and alkyl polyoxyethylene ethers can be used as the anti-rust agent. In a case where the lubricating oil composition contains an anti-rust agent, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition. 
     Known metal deactivators such as imidazolines, pyrimidine derivatives, alkylthiadiazoles, mercaptobenzothiazoles, benzotriazoles and their derivatives, 1,3,4-thiadiazole polysulfide, 1,3,4-thiadiazolyl-2,5-bis(dialkyl dithiocarbamate), 2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio)propionitrile can be used as the metal deactivator. In a case where the lubricating oil composition contains a metal deactivator, the content thereof is usually 0.005 to 1 mass % on the basis of the total mass of the lubricating oil composition. 
     Known demulsifiers such as polyalkylene glycol-based nonionic surfactants can be used as the demulsifier. In a case where the lubricating oil composition contains a demulsifier, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition. 
     Known anti-foaming agent such as silicones, fluorosilicones, and fluoroalkyl ethers can be used as the anti-foaming agent. In a case where the lubricating oil composition contains an anti-foaming agent, the content thereof is usually 0.0001 to 0.1 mass % on the basis of the total mass of the lubricating oil composition. 
     As the coloring agent, for example, a known coloring agent such as azo compounds can be used. 
     &lt;Lubricating Oil Composition&gt; 
     The kinematic viscosity of the lubricating oil composition at 100° C. is preferably 4.0 to 12 mm 2 /s, more preferably no more than 9.3 mm 2 /s, especially preferably no more than 8.5 mm 2 /s; and more preferably no less than 5.0 mm 2 /s, further preferably no less than 5.5 mm 2 /s, especially preferably no less than 6.1 mm 2 /s. In a case where the kinematic viscosity of the lubricating oil composition at 100° C. is under 4.0 mm 2 /s, lubricity might be insufficient. In a case where the kinematic viscosity thereof is beyond 12 mm 2 /s, necessary low-temperature viscosity and sufficient fuel efficiency might not be obtained. 
     The kinematic viscosity of the lubricating oil composition at 40° C. is preferably 4.0 to 50 mm 2 /s, more preferably no more than 40 mm 2 /s, especially preferably no more than 35 mm 2 /s; and more preferably no less than 15 mm 2 /s, further preferably no less than 18 mm 2 /s, especially preferably no less than 20 mm 2 /s. In a case where the kinematic viscosity of the lubricating oil composition at 40° C. is under 4 mm 2 /s, lubricity might be insufficient. In a case where the kinematic viscosity thereof is beyond 50 mm 2 /s, necessary low-temperature viscosity and sufficient fuel efficiency might not be obtained. 
     The viscosity index of the lubricating oil composition is preferably 140 to 400, more preferably no less than 60, further preferably no less than 180, especially preferably no less than 200, and most preferably no less than 210. In a case where the viscosity index of the lubricating oil composition is under 140, it might be difficult to improve fuel efficiency while keeping the HTHS viscosity at 150° C., and further, to reduce low-temperature viscosity (for example, the viscosity at −35° C. that is measurement temperature of the CCS viscosity specified in SAE viscosity grade 0W-X, known as a viscosity grade of fuel-efficient oils). In a case where the viscosity index of the lubricating oil composition is beyond 400, evaporation loss might deteriorate, and further, malfunctioning might occur due to insufficient solubility of additives and compatibility with sealing materials. 
     The HTHS viscosity of the lubricating oil composition at 100° C. is preferably no more than 5.5 mPa·s, more preferably no more than 5.0 mPa·s, especially preferably no more than 4.8 mPa·s; and preferably no less than 3.0 mPa·s, more preferably no less than 3.5 mPa·s, especially preferably no less than 4.0 mPa·s. In this specification, the HTHS viscosity at 100° C. indicates high temperature high shear viscosity at 100° C., specified in ASTM D4683. In a case where the HTHS viscosity at 100° C. is under 3.0 mPa·s, lubricity might be insufficient. In a case where the HTHS viscosity at 100° C. is beyond 5.5 mPa·s, necessary low-temperature viscosity and sufficient fuel efficiency might not be obtained. 
     The HTHS viscosity of the lubricating oil composition at 150° C. is preferably no more than 2.7 mPa·s, more preferably no more than 2.4 mPa·s; and preferably no less than 1.9 mPa·s, more preferably no less than 2.1 mPa·s. In this specification, the HTHS viscosity at 150° C. indicates high temperature high shear viscosity at 150° C., specified in ASTM D4683. In a case where the HTHS viscosity at 150° C. is under 1.9 mPa·s, lubricity might be insufficient. In a case where the HTHS viscosity at 150° C. is beyond 2.7 mPa·s, fuel efficiency might be insufficient. 
     The evaporation loss of the lubricating oil composition is, as NOACK evaporation loss at 250° C., preferably no more than 30 mass %, further preferably no more than 20 mass %, and especially preferably no more than 15 mass %. In a case where the NOACK evaporation loss of the lubricating oil composition is beyond 30 mass %, the evaporation loss of the lubricating oil is large, which causes viscosity increase and the like, and is thus unfavorable. In this specification, the NOACK evaporation loss is evaporation loss of the lubricating oil measured conforming to ASTM D 5800. The lower limit of the NOACK evaporation loss of the lubricating oil composition at 250° C. is not restricted, but normally no less than 5 mass %. 
     The mass ratio (MB/Mg) of the boron content (MB) to the magnesium content (Mg) in the lubricating oil composition is 0.5 to 10, preferably no less than 0.8, and preferably no more than 8. The mass ratio MB/Mg of the above lower limit or over makes it possible to improve fuel efficiency. The mass ratio MB/Mg of the above upper limit or below makes it possible to maintain fuel efficiency. 
     The lubricating oil composition of the present invention satisfies at least one of the following requirements (i) to (iii): 
     (i) the boron content in the composition is no less than 270 mass ppm on the basis of the total mass of the composition; 
     (ii) the component (C) contains a boric acid salt-overbased metallic detergent (that may compose at least a part of the component (B1) and/or the component (B2)); and 
     (iii) the mass ratio (MB/Mg) of the boron content (MB) to the magnesium content (Mg) in the composition is no less than 0.8. 
     Satisfying at least one of the above requirements (i) to (iii) makes it possible to improve fuel efficiency. 
     EXAMPLES 
     Hereinafter the present invention will be more specifically described based on examples and comparative examples. It is noted that the present invention is not limited to these examples. 
     Examples 1 to 8 and Comparative Examples 1 to 5 
     Each of the lubricating oil compositions of the present invention (examples 1 to 8) and the lubricating oil compositions for comparison (comparative examples 1 to 5) was prepared using the following base oil and additives. Formulation of each composition is shown in Table 2. In Table 2, “mass %” means mass % on the basis of the total mass of each composition, “mass ppm” means mass ppm on the basis of the total mass of each composition, and “mass ratio” means a ratio by mass. 
     (Base Oil) A-1: a hydrocracked base oil having properties shown in Table 1. In Table 1, “mass ppm” means mass ppm on the basis of the total mass of the base oil, and “mass %” means mass % on the basis of the total mass of the base oil. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Properties 
                 Unit 
                 Value 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Density (15° C.) 
                 g/cm 3   
                 0.820 
               
               
                   
                 Kinematic Viscosity (40° C.) 
                 mm 2 /s 
                 17.8 
               
               
                   
                 Kinematic Viscosity (100° C.) 
                 mm 2 /s 
                 4.07 
               
               
                   
                 Viscosity Index 
                   
                 132 
               
               
                   
                 Pour Point 
                 ° C. 
                 −22.5 
               
               
                   
                 Aniline Point 
                 ° C. 
                 119 
               
               
                   
                 Iodine Number 
                   
                 0.05 
               
               
                   
                 S Content 
                 mass ppm 
                 &lt;1 
               
               
                   
                 N Content 
                 mass ppm 
                 &lt;3 
               
               
                   
                 n-d-M Analysis 
               
               
                   
                 % C P   
                   
                 87.3 
               
               
                   
                 % C N   
                   
                 12.7 
               
               
                   
                 % C A   
                   
                 0 
               
               
                   
                 Chromatographic Analysis 
               
               
                   
                 Saturated Content 
                 mass % 
                 99.6 
               
               
                   
                 Aromatic Content 
                 mass % 
                 0.2 
               
               
                   
                 Resin Content 
                 mass % 
                 0.2 
               
               
                   
                 Recovery Rate 
                 mass % 
                 100 
               
               
                   
                   
               
            
           
         
       
     
     (Metallic Detergents) 
     B1-1: calcium carbonate-overbased calcium salicylate, Ca content: 6.2 mass %, metal ratio: 2.3, alkyl chain length: 14-18, base number (perchloric acid method): 180 mgKOH/g. 
     B1-2 (C1): calcium borate-overbased calcium salicylate, Ca content: 6.8 mass %, boron content: 2.7 mass %, metal ratio: 2.5, and base number (perchloric acid method): 190 mgKOH/g. 
     B1-3 (C1): calcium borate-overbased calcium salicylate, Ca content: 5.0 mass %, boron content: 1.8 mass %, metal ratio: 1.5, base number (perchloric acid method): 140 mgKOH/g. 
     B2-1: magnesium carbonate-overbased magnesium sulfonate, Mg content: 9.5 mass %, base number (perchloric acid method): 400 mgKOH/g, sulfur content: 2 mass %. 
     (Ashless Dispersants) 
     C 2 a′-1: polybutenylsuccinimide, Mw: 9000, nitrogen content: 0.7 mass % boron content: 0 mass %. 
     C 2 a-1: boronated polybutenylsuccinimide, Mw: 6000, nitrogen content: 1.6 mass % boron content: 0.5 mass %. 
     (Oil-Soluble Organic Molybdenum Compounds) 
     D-1: sulfurized (oxy)molybdenum dithiocarbamate 
     D-2: Mo-based anti-oxidant 
     (Viscosity Index Improver) 
     E-1: non-dispersant polymethacrylate viscosity index improver, in weight average molecular weight: 400,000, PSSI: 25 
     (Other Additives) 
     F-1: additive mixture containing a zinc dialkyl dithiophosphate, an ashless anti-oxidant, and a anti-foaming agent. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 Examples 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                   
               
               
                 Base oil 
               
               
                 A-1 
                   
                 balance 
                 balance 
                 balance 
                 balance 
                 balance 
                 balance 
                 balance 
               
               
                 Metallic detergents 
               
               
                 B1-1 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 1.5 
                 — 
               
               
                 B1-2(C1-1) 
                 mass % 
                 2.3 
                 — 
                 2.3 
                 1.7 
                 1.7 
                 0.8 
                 2.3 
               
               
                 B1-3(C1-2) 
                 mass % 
                 — 
                 2.9 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 B2-1 
                 mass % 
                 0.2 
                 0.2 
                 0.08 
                 0.4 
                 0.2 
                 0.3 
                 0.2 
               
               
                 Ashless dispersants 
               
               
                 C2a′-1 
                 mass % 
                 3.2 
                 3.2 
                 3.2 
                 3.2 
                 — 
                 3.2 
                 3.2 
               
               
                 C2a-1 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 3.0 
                 — 
                 — 
               
               
                 Oil-soluble organic Mo compounds 
               
               
                 D-1 (in terms of Mo) 
                 mass ppm 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
               
               
                 D-2 (in terms of Mo) 
                 mass ppm 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
               
               
                 Total Amount (D-1 + D-2) 
                 mass % 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
               
               
                 Viscosity index improver 
               
               
                 E-1 
                 mass % 
                 5.3 
                 5.3 
                 5.3 
                 5.3 
                 5.3 
                 5.3 
                 7.7 
               
               
                 Other additives 
               
               
                 F-1 
                 mass % 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
               
               
                 Kinematic viscosity 
               
               
                 40° C. 
                 mm 2 /s 
                 26.4 
                 26.5 
                 26.5 
                 26.5 
                 26.5 
                 26.5 
                 28.7 
               
               
                 100° C. 
                 mm 2 /s 
                 6.5 
                 6.5 
                 6.5 
                 6.5 
                 6.5 
                 6.5 
                 7.7 
               
               
                 Viscosity index 
                   
                 214 
                 214 
                 214 
                 214 
                 214 
                 214 
                 235 
               
               
                 HTHS Viscosity 
               
               
                 100° C. 
                 mPa · s 
                 4.5 
                 4.5 
                 4.5 
                 45 
                 4.5 
                 4.5 
                 4.8 
               
               
                 150° C. 
                 mPa · s 
                 2.3 
                 2.3 
                 2.3 
                 2.3 
                 2.3 
                 2.3 
                 2.6 
               
               
                 NOACK evaporation loss (250° C., 1 
                 mass % 
                 12 
                 12 
                 12 
                 12 
                 12 
                 12 
                 12 
               
               
                 Elements in Oil 
               
               
                 Ca 
                 mass ppm 
                 1600 
                 1600 
                 1600 
                 1200 
                 1600 
                 1500 
                 1600 
               
               
                 B 
                 mass ppm 
                 600 
                 500 
                 600 
                 450 
                 600 
                 200 
                 600 
               
               
                 Mg 
                 mass ppm 
                 240 
                 240 
                 100 
                 500 
                 240 
                 350 
                 240 
               
               
                 Mo 
                 mass ppm 
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
               
               
                 B/Mg 
                 mass ratio 
                 2.5 
                 2.1 
                 6 
                 0.9 
                 2.5 
                 0.6 
                 2.5 
               
               
                 Stand-alone valve-train lest 
               
               
                 Torque reduction (vs. Comp. Ex. 
                 % 
                 2.8 
                 5.2 
                 2.2 
                 2.0 
                 1.5 
                 0.9 
                 2.8 
               
               
                 Motoring ergine torque test 
               
               
                 Torque reduction (vs. Comp. Ex. 
                 % 
                 2.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 1.6 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Examples 
                 Comparative examples 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 8 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
                   
               
               
                   
                 Base oil 
               
               
                   
                 A-1 
                   
                 balance 
                 balance 
                 balance 
                 balance 
                 balance 
                 balance 
               
               
                   
                 Metallic detergents 
               
               
                   
                 B1-1 
                 mass % 
                 — 
                 2.4 
                 3.1 
                 1.8 
                 2.2 
                 1.8 
               
               
                   
                 B1-2(C1-1) 
                 mass % 
                 2.3 
                 — 
                 — 
                 — 
                 0.2 
                 0.8 
               
               
                   
                 B1-3(C1-2) 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 B2-1 
                 mass % 
                 0.2 
                 0.2 
                 — 
                 0.4 
                 0.2 
                 0.7 
               
               
                   
                 Ashless dispersants 
               
               
                   
                 C2a&#39;-1 
                 mass % 
                 3.2 
                 3.2 
                 3.2 
                 3.2 
                 3.2 
                 3.2 
               
               
                   
                 C2a-1 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 Oil-soluble organic Mo compounds 
               
               
                   
                 D-1 (in terms of Mo) 
                 mass ppm 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
                 (800) 
               
               
                   
                 D-2 (in terms of Mo) 
                 mass ppm 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
                 (200) 
               
               
                   
                 Total Amount (D-1 + D-2) 
                 mass % 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
               
               
                   
                 Viscosity index improver 
               
               
                   
                 E-1 
                 mass % 
                 — 
                 5.3 
                 5.3 
                 5.3 
                 5.3 
                 5.3 
               
               
                   
                 Other additives 
               
               
                   
                 F-1 
                 mass % 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
               
               
                   
                 Kinematic viscosity 
               
               
                   
                 40° C. 
                 mm 2 /s 
                 19 
                 26.5 
                 26.4 
                 36.4 
                 26.5 
                 26.5 
               
               
                   
                 100° C. 
                 mm 2 /s 
                 4.9 
                 6.5 
                 6.5 
                 6.5 
                 6.5 
                 6.5 
               
               
                   
                 Viscosity index 
                   
                 151 
                 214 
                 214 
                 215 
                 214 
                 214 
               
               
                   
                 HTHS Viscosity 
               
               
                   
                 100° C. 
                 mPa · s 
                 3.9 
                 4.5 
                 4.5 
                 4.5 
                 4.5 
                 4.5 
               
               
                   
                 150° C. 
                 mPa · s 
                 1.9 
                 2.3 
                 2.3 
                 2.3 
                 2.3 
                 2.3 
               
               
                   
                 NOACK evaporation loss (250° C., 1 
                 mass % 
                 12 
                 12 
                 12 
                 12 
                 12 
                 12 
               
               
                   
                 Elements in Oil 
               
               
                   
                 Ca 
                 mass ppm 
                 1600 
                 1600 
                 1600 
                 1200 
                 1600 
                 1600 
               
               
                   
                 B 
                 mass ppm 
                 600 
                 0 
                 0 
                 0 
                 30 
                 150 
               
               
                   
                 Mg 
                 mass ppm 
                 240 
                 240 
                 0 
                 500 
                 150 
                 700 
               
               
                   
                 Mo 
                 mass ppm 
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
               
               
                   
                 B/Mg 
                 mass ratio 
                 2.5 
                 0 
                 0 
                 0 
                 0.2 
                 0.2 
               
               
                   
                 Stand-alone valve-train lest 
               
               
                   
                 Torque reduction (vs. Comp. Ex. 
                 % 
                 2.9 
                 reference 
                 −0.8 
                 0.0 
                 −2.1 
                 −3.0 
               
               
                   
                 Motoring ergine torque test 
               
               
                   
                 Torque reduction (vs. Comp. Ex. 
                 % 
                 4.8 
                 reference 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     (Stand-Alone Valve-Train Test) 
     For each lubricating oil composition of examples 1 to 8 and comparative examples 1 to 5, low friction performance was evaluated by a valve-train system motoring friction test machine. 
     A valve-train system motoring friction test machine is a device which can measure friction torque of a pair of a cam and a tappet in a valve-train system of a direct acting engine. Friction torque at 80° C. in oil temperature at 350 rpm in rotation frequency was measured while the machine was lubricated by each lubricating oil composition. Then, the torque reduction rate compared to the measurement value in the comparative example 1 was calculated. As the reduction rate is higher, fuel efficiency is better. Results are shown in Table 2. 
     (Motoring Engine Torque Test) 
     For each lubricating oil composition of examples 1, 7 and 8 and comparative example 1, a motoring engine torque test was further carried out. For each lubricating oil composition, torque necessary for rotating an output shaft of a DOHC engine (displacement: 2 L) by an electric motor at certain rotation frequency was measured, while the engine was lubricated by each lubricating oil composition (oil temperature: 80° C.). The measurement was performed at 1400 rpm, and the torque reduction rate compared to the measurement value in the comparative example 1 was calculated. As the torque reduction rate is higher, fuel efficiency is better. Results are shown in Table 2. 
     INDUSTRIAL APPLICABILITY 
     The lubricating oil composition of the present invention makes it possible to secure LSPI suppression performance and detergency, and to improve fuel efficiency at the same time. Thus, the lubricating oil composition of the present invention can be preferably used for lubrication of turbocharged gasoline engines, especially turbocharged direct injection engines, which is apt to suffer LSPI problems.