The Impact of Passenger Car Motor Oil Phosphorus Levels on Engine Durability, Oil Degradation, and Exhaust Emissions in a Field Trial

A 100,000-mile fleet test was carried out on nine 1991 gasoline-powered passenger cars employing an API SH/CD motor oil and two reduced phosphorus analogues. The lower-phosphorus oils have zinc dialkyldithiophosphate (ZDDP) treat rates that fall below the proposed ILSAC GF-2 maximum phosphorus limit (0.11%). Gaseous tail pipe emissions were measured at various intervals according to the EPA FTP City Emissions Test 75 driving cycle. A good correlation between phosphorus level and emissions degradation was obtained when starting emissions levels and oil consumption was accounted for in the analysis.

Few differences were observed between the highest-phosphorus oil (0.11%) and the lower-phosphorus (0.08% and 0.06%) oils in the typical end of test engine cleanliness parameters. There were no significant differences in either valve train or cylinder wear between the oils. The used oils had similar analytical inspections. However, the 0.11% phosphorus oil did have a lower average viscosity increase than the 0.06% phosphorus oil. This was ascribed to lower oxidative thickening for this oil.