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Because of the rising costs of engine tests, bench testing is a necessity in engine oil development. Which bench test to use remains a problem. Recently, we have reported on the use of electrical contact resistance (ECR) coupled with a ball-on-disk tribometer to study the formation and the durability of antiwear films from binary additive mixtures. This paper extends the ECR study to fully formulated fresh oils run in both fired gasoline engines and the ECR bench test. X-ray Photoelectron Spectroscopy (XPS) analyses of used Sequence VE engine parts from highash fully formulated lubricants are shown and the relationship of ECR film formation to fired-engine test performance is discussed.

Modern Passenger Car Motor Oils (PCMOs) are formulated to provide superior wear, oxidation, and deposit control under the most demanding driving conditions. In most PCMOs, zinc dialkyldithiophosphate (ZnDTP) has been the dominant antiwear and antioxidant agent for well over 50 years. Unfortunately, the phosphorus in ZnDTP may partially deactivate the exhaust emission catalyst. To ensure that the catalysts function for at least 120,000 miles, engine manufacturers are exploring phosphorus limitations for upcoming PCMO performance categories. This paper explores the antiwear film formation of low phosphorus engine oils using the Electrical Contact Resistance bench test. A prototype ILSAC GF-4 formulation blended with varying amounts and types of ZnDTP was tested at temperatures typical of operating engines. Secondary ZnDTP was found to produce the best films under the broadest temperature range.