The mechanism of overhead valve train wear in moderate to low temperature service was studied using a modified fired V-D test and a motored V-D cam and cam-follower rig. High wear and Sow wear used oils from the fired test gave the correct relative wear in the motored test, indicating the motored test is a valid tool for studying wear mechanisms. Key factors affecting valve train wear were isolated and selectively introduced in a series of motored engine tests. Results from this study showed the expected increase in wear with a decrease in viscosity of unformulated lubricants. Added zinc diaikyldithiophosphate (ZDDP) reduced wear in a low viscosity lubricant and a used oil as anticipated. A high detergent, high wear oil, in an unused state, did not produce significant wear in the motored test even if all of the ZDDP was removed. Significant wear resulted only after exhaust gases (simulated blowby) were fed into the motored engine sump containing the high wear oil. Laboratory simulation of blowby effects showed the importance of wear resulting from oil aging by chemical reactions between the lubricant and blowby gases. This effect is important even when the viscosity of the lubricant is otherwise sufficient to preclude wear. Active ZDDP depletion by thermal and oxidative routes contributes to wear. Viscosity losses in the Sequence V-D test and in the fired test were large due to fuel dilution and Viscosity Index Improver shear which can lead to further increases in wear.