Engine technology in passenger vehicles continues to evolve to meet consumer needs, improve fuel efficiency and reduce emissions. Likewise, engine oil classifications and performance requirements have evolved to meet the demands of the latest engine technology. This paper explores the degradation of engine oil quality in service by monitoring the condition of the used oil samples obtained from real-world, non-fleet consumer applications. In a previous study (SAE
2003-01-1957), used oil analysis results were reported for API/ILSAC GF-2 engine oil samples to determine oil degradation trends during service life. These samples were collected from a variety of vehicles, representing different driving conditions and oil brands. Each sample was analyzed to monitor additive depletion, accumulation of contaminants, and change in the physical and chemical properties of the oils. This study is a continuation of the previous work and includes API/ILSAC GF-4 engine oil samples. The data indicates an improvement in the used oil condition from API/ILSAC GF-2 to API/ILSAC GF-4 oils. As indicated in certain vehicles' manuals, several factors including individual driving habits and local operating conditions can impact how motor oil performs and degrades in service. These findings indicate that although engine and engine oil technology continue to improve, it is not possible to predict oil's expected life in service at statistically significant levels under a variety of operating conditions and external variables based upon engine oil age (miles). There appear to be circumstances in the current passenger vehicle population that would necessitate engine oil changes at relatively short drain intervals.