Engine wear, particularly that of the valve train, was examined in motored 2.2L and 2.3L engines. Procedures were developed which involved addition of synthetic hydroperoxide to the crank-case oil to simulate fired engine conditions. Cam lobe and follower wear occurred, apparently as a result of direct attack by the hydroperoxide on rubbing surfaces. This type of hydroperoxide-induced wear was identified in a fired bench engine test and in a field test. Similar hydroperoxide-induced wear was also observed in the lesser loaded engine contact areas such as bearings. Phosphorus nuclear magnetic resonance (NMR) has proven to be a useful tool to the understanding of the structure and antioxidant antiwear properties of ZnDDP and its interactions with dispersant/detergent combinations.