Oil formulation has been found to be a significant factor in high rates of 6.2 L diesel engine, roller hydraulic valve lifter wear that occurred in field service with some commercial engine oils. This was confirmed through engine-dynamometer testing. A correlation has been established between engine-dynamometer wear test results and those obtained in laboratory four-ball wear tests conducted with used engine oil. The effects of dispersant level, viscosity, sulfonate metal type, sulfonate total-base-number, zinc dialkyl dithiophosphate (ZDTP) type, and ZDTP concentration on wear were systematically investigated. Wear increased with increasing soot concentration in the oil, and decreased with increasing sulfur concentration, both in the oil and on the metal surface. Wear also decreased with increasing dispersant concentration. The remaining oil variables had minimal effects on wear within the ranges studied. Diesel engine wear occurs when soot either prevents antiwear film formation on metal surfaces, or removes the antiwear film shortly after formation.