Fuel-efficient lubricating oil
This is a continuation of application Ser. No. 357,038, filed on Mar. 11, 1982 and which is now abandoned, which, in turn, is a continuation of Ser. No. 206,516 filed Nov. 13, 1980 and which is now abandoned, which, in turn, is a continuation of Ser. No. 968,528 filed Dec. 11, 1978 and which is now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method for improving the fuel economy of an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     With the crisis associated with diminishing amounts of fossil fuel and the rapidly increasing prices for this fuel, there has been a great deal of interest in improving fuel economy, that is, the number of miles obtained in a given vehicle per liter of fuel. 
     Pentaerythritol mono-oleate is an additive which has been widely used, especially in the industrial oil area, and at low concentrations, that is less than about one-half of one percent, in the industrial oils. It is known to be an oiliness agent; however, since piston and ring lubrication is predominantly hydrodynamic, gains in fuel economy through the use of additives in the lubricating oil which reduce the coefficient of friction in mixed lubrication will probably be small and difficult to assess. 
     SUMMARY OF THE INVENTION 
     It has now been found that if 0.25 to 2 weight of pentaerythritol mono-oleate in combination with 15 to 25 millimols per kilogram of zinc O,O-di(2-ethylhexyl)phosphorodithioate is added to the lubricating oil used to lubricate the crankcase of an internal combustion engine a measurable improvement in the fuel economy, that is, the miles per liter of fuel, of the engine is observed without any degradation in corrosion performance as measured by the L-38 engine test. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Adding from 0.25 to 2 weight percent, and preferably from 0.40 to 1.25 weight percent of pentaerythritol mono-oleate along with 15 to 25 millimols per kilogram of zinc O,O-di(2-ethylhexyl)phosphorodithioate to a crankcase lubricating oil significantly improves the fuel economy of the internal combustion engine. Specifically, improvements in fuel milage of from 2 to 3% on the average have been observed in engine tests. This fuel economy improvement has been observed for both compression-ignition engines, that is, diesel engines, and spark-ignition engines, that is, gasoline engines. 
     Pentaerythritol mono-oleate as it is commercially available is usually a mixture of mono-, di-, tri-, and tetraoleates of pentaerythritol. Some oleic acid may also be present. As used herein, the term &#34;pentaerythritol mono-oleate&#34; is intended to include both pure pentaerythritol mono-oleate and mixtures of pentaerythritol mono-, di-, tri-, and tetraoleate. 
     The lubricating oil to which the pentaerythritol mono-oleate can be added can be any hydrocarbon-based lubricating oil. The hydrocarbon lubricating oils may be derived from synthetic or natural sources and may paraffinic, napthenic or asphaltic base, or mixtures thereof. A variety of additives are ordinarily present in lubricating oils used to lubricate modern internal combustion engines. These additives include antioxidants, dispersants, rust inhibitors, foam inhibitors, corrosion inhibitors, antiwear agents, and a variety of other well-known additives.