Viscosity decrease as a function of the duration of mechanical shearing is described by a mathematical model. Applicability of the model to several different types of service is demonstrated. For automobile engine oil service that shows a viscosity decrease initially, followed by a gradual viscosity increase, the model offers a means of separating the viscosity decrease term from the total viscosity change. The model provides a rational method of comparing shearing in bench tests with shearing in service. It also offers a means for detailed comparisons of the shear stability of different lubricants in a given service.