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The effect of phosphorus concentration in gasoline engine oils on the valve train wear was experimentally investigated by using the JASO M328-91 3A valve train wear (3A-VTW) test method. The phosphorus concentration is determined proportionally to the amount of zinc dithiophosphate (ZDDP), which is formulated as both antiwear agent and antioxidant. Lower concentrations of ZDDP generally bring about larger wear in the valve train. However, it was found from the experiments that valve train wear remained low despite a decrease of phosphorus concentration when secondary ZDDPs with short alkyl chain together with appropriate ashless dispersants were selected. Since adsorptivity of secondary ZDDPs with short alkyl chain lengths onto rubbing metal surfaces is higher than that of primary types, the secondary types give excellent antiwear characteristics.

The influence of engine oil viscosity on the wear of piston rings and cam faces has been investigated by fired engine tests using a radioisotope (RI) tracer technique. High-temperature and high-shear-rate (HTHS; 150°C, 1O6 s-1) viscosities of the experimental oils prepared are 2.2, 2.4, 2.6 and 3.1 mPa•s. At an oil temperature of 90°C the wear of piston rings and cam faces did not increase, even if the HTHS viscosity was lowered down to 2.2 mPa•s. However, both piston rings and cam faces exhibited an increase in wear below 2.4 mPa•s at 130°C. It was also recognized that valve train wear did not significantly increase with reducing viscosity in the motored engine tests at a temperature of 50°C. From these test results, it was suggested that the oil with the HTHS viscosity of 2.6 mPa•s sufficiently demonstrates the antiwear performance equivalent to that with around 3.0 mPa•s for application to piston rings and cam faces.

Fuel economy is one of the most important performance features for modern engine oils. For some time now, fuel efficient engine oils (called Energy Conserving II or EC-II) have been available in the marketplace. However, the performance of EC-II oils is only 2.7% Equivalent Fuel Economy Improvement (EFEI) as measured by the ASTM Sequence VI Engine Test. To meet future industry needs, more fuel efficient engine oils are desirable. In order to achieve this, a study of highly fuel efficient engine oils was initiated. An initial target of 3.9% EFEI was selected and several candidate oils were evaluated, some of which exceeded this target. The oils were evaluated using a chassis dynamometer using the U. S. EPA mode. The test results may be summarized: 5W-30 Prototype Oil containing MoDTC showed between 1.6 and 2.6% better fuel economy than conventional 5W-30 and 10W-30 EC-II oils. There was an optimum viscosity for maximum fuel economy using the EPA testing mode.