Extending engine-oil-change intervals is of interest from the standpoint of reducing used oil disposal and reducing time and expense of maintenance. However, the oil must be changed before serious oil degradation and engine damage occur. Three variables which influence oil degradation were chosen for investigation: base oil composition (synthetic oil versus mineral oil), trip length (short trips versus long trips), and driving schedule (degrading an oil during a given type of service, then changing to another type of service without an intervening oil change). Analysis of oil samples taken throughout the testing program indicated that type of service (freeway compared to short trip) influenced oil degradation to a greater extent than oil type. That is, API SG-quality synthetic oil in short-trip service degraded faster than borderline SG-quality mineral oil in long-trip service. Inspection at the end of the series of tests indicated that there had not been any significant damage to the engine, despite extended oil-drain periods. Data from the short-trip portion of these tests support the recommended oil-change intervals published in current owner's manuals (typically 5000 km or three months). Under long-trip driving conditions in which recommended oil-change intervals are typically 12,000 km or one year, the data of this study suggest that extended drain intervals may be possible.