Patent ID: 8642523

Claim:
A method for improving the fuel economy of large low and medium speed engines that reach surface speeds of at least 10 mm/s and are lubricated by an engine oil by reducing the traction coefficient of the engine oil used to lubricate the engine by employing as the engine oil a lubricating oil having a kinematic viscosity at 100° C. of 8 to 25 mm 2 /s, comprising a base oil comprising a bimodal blend of two different base oils, the first base oil being one or more oils selected from Group III base oil and Group IV base oil, which first base oil has a kinematic viscosity at 100° C. of 2 to 12 mm 2 /s and a second base oil selected from one or more polyisobutylenes (PIBs) having a number average molecular weight of at least 1300 Mn and kinematic viscosity at 100° C. of at least 630 mm/s 2 , and a detergent present in the range 8 to 25 wt % based on active ingredient selected from a mixture of alkali and/or alkaline earth metal phenate and salicylate, or a mixture of alkali and/or alkaline earth metal phenate and carboxylate with a total base number ranging from 40 to 400 mg KOH/g, wherein the improvement in fuel economy is evidenced by the engine oil having a traction coefficient which is lower than the traction coefficient of engine oils which are not bimodal or which are based on one or more Group I base stocks or on a mixture of Group I base stocks and PIB.