Engine design and operational changes have led to increased motor oil temperatures. High oil temperatures have resulted in scattered instances of excessive motor oil viscosity increase in service. In the most severe oil thickening cases the oil would not flow, which caused engine failure. Trends in engine design including the incorporation of additional emission control devices, are expected to aggravate this potential problem further.A laboratory engine test technique has been developed which evaluates the performace of motor oils under high temperature operation. This technique was used to differentiate the various compositional parameters affecting oil oxidation and subsequent oil thickening. Insoluble oxidation products were responsible for most of the viscosity increase. The choice of blending stocks was significant to the stability. The influence of additives was minor.