The effect of pure fuels and several mixtures of pure fuels on lubricant degradation rates was determined by burning them in a CLR engine fitted with a Cu-Pb bearing and operated at high speeds and high crankcase temperatures. Oil degradation rates were measured by infrared analysis and the rate of corrosion of the Cu-Pb bearing by oil analysis for copper. Using a typical API-SD SAE 20 grade motor oil, fuels stressed the lubricant in the following decreasing order: diisobutylene >isooctane >decene-1 >hexene-l >cumene >benzene = tert-butylbenzene >> xylenes >toluene. The lubricant responded to mixtures of these fuels in a manner predictable from the response to pure fuels. The generally low stresses applied to the lubricant by the aromatic fuels may be attributed, at least in part, to the antioxidant action of phenols formed during combustion. No mechanism was established for the markedly lower stresses observed with the methyl-substituted aromatics.