To assist in the development of a bench shear stability test, computer models of shear degradation and thermal/oxidation (T/O) degradation processes of VI improvers in engine oils have been developed. The effects of the three elements of the process: VI improver, base fluid, and degradation device were separated. The models include adjustable parameters for each element. The models then allow physical degradation devices to be characterized by adjusting the parameters of the model of the device until the molecular weight distributions match the distributions produced by the device. Attempts were made to characterize the ASTM D 3945 test this way using molecular weight distributions as determined by gel permeation chromatography. This was done for two different types of VI improver. However, the shear device characteristics which produced a good match for one did not produce a good match for the other, and vice versa. Attempts were also made to characterize a specific engine as a T/O degradation device with one VI improver and to characterize the engine as a combination of shear and T/O devices with another. A computer model is proposed in which multiple shear and T/O degradation devices are combined in a parallel/serial array. This single model will allow simulation of a simple bench test or a complete engine with multiple oil flow paths.