Recent trends in engine design and automotive service have led to increased engine oil temperatures and, as a result, to occasional reports of excessive oil thickening in service. The thickened oils are usually very highly oxidized.Laboratory engine and bench tests have been developed to evaluate three parameters which affect oil thickening. The role of chemical inhibitors, base-oil volatility, and moderate temperature service were evaluated. Chemical inhibitors were effective in controlling excessive oxidation, but significant thickening may occur because of base oil volatilization. Prolonged service at moderate temperatures depleted the ability of an oil to resist excessive oxidation at high temperatures.