The average oil change interval for passenger vehicles in the USA is gradually increasing, and is currently approaching 8,320 km (5,200 miles). This paper details the results of lubricant condition monitoring on samples taken from hundreds of vehicles at intervals ranging from 0 to 25,600 km (16,000 miles). The data indicate steady additive depletion by 4,800 to 9,600 km (3,000-6,000 miles), resulting in a concomitant decrease in measured oxidation resistance. Oxidation and nitration of the basestock was also found to be present at this point, resulting in a gradual increase in both kinematic and HTHS viscosity. As a result, it is predicted that excessively long drain intervals will produce a measurable increase in fuel consumption and associated CO2 emissions. Many owners' manuals recommend service intervals of 12,000 and 4,800 km (7,500 and 3,000 miles) under “normal” and “severe” service conditions, respectively. Overall, the data indicate that the majority of passenger vehicles operate under “severe” service conditions. This finding is confirmed by a survey of owners, many of which are unaware that “normal” service does not include stop and go traffic, short trips, etc. However, the data indicate that longer drain intervals are possible for lubricants formulated using synthetic basestocks.