Current emphasis on fuel economy has evoked the desire to reduce lubricant viscosity both to improve fuel mileage and make starting easier with lighter starter systems. The present study was conducted to determine the effect of viscosity on wear under conditions associated with boundary lubrication. Measurements were obtained in a laboratory test designed to simulate the camshaft-lifter contact in an automobile or light truck engine. Viscosity reduction was shown to have a profound effect on wear once a critical viscosity had been reached. In the simulated camshaft-lifter contact, wear prevention appears to be due to a mixture of localized elastohydrodynamic lubrication and surface films formed from the antiwear additive. Loss of either results in excessive wear. The break in the wear-viscosity relationship occurred at 4–5 cSt @ 100 °C. Since this viscosity corresponds to the low end of the SAE 10 grade, it is possible that a 7-1/2W or 5W oil could result in excessive wear. These results suggest that any proposed low viscosity oil should undergo careful testing for its wear properties.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Dayton, Ohio, October 16–18, 1979