The trend to longer oil drain intervals for passenger cars has hastened in the last few years. Most earlier studies on the effects of extended drains on oil and engine performance were conducted prior to 1973 when significant engine modifications were made to reduce exhaust emissions. This paper describes several taxi and other road tests, supplemented by laboratory programs, conducted since 1973. The total field program included approximately 225 cars over sixteen million kilometers. All of the work reported is with 5W and 10W multigrade engine oils.
Taxi test results have shown sludge build-up and oil thickening to be the most significant failure modes as drains are extended beyond approximately 12 000 km. This occurs even with oils giving excellent Sequence IIIC and VC results. Oil thickening and particularly engine sludge and filter plugging were more severe in winter driving and with leaded gasoline. With conventional 5W multigrade SE oils, engine wear, particularly in the cam and lifter region, was found to be very dependent on test conditions and was severe in some. However, with extended drain additive technology, excellent overall performance has been demonstrated with 5W30 engine oils at 24 000 km oil drains under summer driving conditions. Used oil corrosivity, measured in the laboratory, and engine wear were found to be relatively independent of used oil acidity measured by Total Acid Number (TAN).
In terms of oil thickening, results from a low temperature starting/crankability program with fresh and used oils suggested that for a 100% increase in CCS viscosity, the minimum starting temperature increased by only 1-3°C. Basestock volatility was found to be critical in controlling low temperature viscosity increases at extended drains. Superior additive technology and low volatility, narrow cut basestocks were both considered necessary for adequate performance at 24 000 km oil drains.