A mechanical shearing test has been developed to evaluate the permanent viscosity loss of multigrade crankcase oils in field service. The test shows two advantages over a sonic shear test used previously. It causes polymer breakdown directionally the same as engines in the field, higher with higher viscosity oils; and test severity does not depend on the polymer type used. The new procedure provides a reliable laboratory technique for designing the used oil viscometric properties of multigrade formulations. Results from a 4 hr test predict the used oil viscosity with and without crankcase dilution, and also the extent of polymer breakdown in field service.