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Engine oils contain additives that provide wear protection to prolong engine life. In a previous study using direct acting mechanical bucket valve train components, we found that aged oil provided better wear protection and friction reduction under certain circumstances. To understand this effect further, friction and wear performance of fresh and laboratory-aged oils with 0.1% phosphorus was studied with ball-on-flat and cylinder-on-flat rigs. Test durations were chosen according to the electrical contact resistance (ECR) values observed between the contacting surfaces. Anti-wear films were characterized primarily by UV and visible Raman spectroscopy, and results were corroborated by Auger electron and infrared spectroscopies. The greatest compositional differences occurred between films formed by fresh and aged oils. The degree of ECR response or the length of oil aging generally did not affect the type of component observed in the films.