This paper discusses the viscosity of viscosity index-improved engine oils in service and outlines a method for predicting this viscosity. Field tests in employee cars were used to determine the extent of polymer breakdown under suburban driving conditions, and this field breakdown was related to the breakdown in the proposed ASTM test for sonic shear stability. The effects of crankcase dilution, base stock oxidation, and oil insolubles on used-oil viscosity are also discussed briefly.