Value train wear was examined in a motored 2.3 l engine similar to that wed in the Sequence V-D test. A procedure was developed in which model hydroperoxides were added to the crankcase oil to simulate used oil. Cam lobe and follower wear occurred, apparently as a result of direct attack by the hydroperoxides on rubbing surfaces rather than through oil oxidation. The extent of wear was proportional to the hydroperoxide concentration. The same type of hydro-peroxide-induced wear subsequently was identified in a fired bench engine and in a field test.