Two current motor oil trends are lower oil volatility and extended oil drain intervals. The reductions in oil consumption accompanying decreased oil volatility provide possible improved exhaust emissions and reduced servicing and maintenance. Longer drain intervals allow reduced frequency of required vehicle service, providing a lower cost of ownership to the consumer. Given these concurrent trends, this study investigates the possible impacts of volatility in extended drain applications.
Four 5W30 oils having volatilities ranging from 5.3 to 21.5 wt % evaporative loss by ASTM 5800 were tested in extended drain New York City taxi service. The drain intervals were 10,000 miles and 20,000 miles, and the test duration was 100,000 miles. The additive packages were of API SJ or SJ+ quality. Oil performance was found to be significantly impacted by volatility. A primary effect of volatility was the degree of oil consumption and resultant replenishment with fresh oil. The used oils for the high volatility formulations showed an enhancement of additive metal concentrations of 20 % to 40 % relative to the initial formulation, whereas the low volatility used oils showed a slight decrease in additive metal concentrations. With the lesser amount of oil additions, the low volatility oils showed a poor control of oxidation, deposit, and wear performance, indicative of additive depletion. However due to loss of light base oil fractions, the high volatility oils showed significant oil thickening. The results suggest that for severe service, extended drain applications oil formulations require a combination of both good volatility and sufficient additive treat to provide good engine cleanliness and wear as well as acceptable levels of oil thickening.