A kinetics problem for a degrading polymer additive dissolved in a base stock is studied. The polymer degradation may be caused by the combination of such lubricant flow parameters as pressure, elongational strain rate, and temperature as well as lubricant viscosity and the polymer characteristics (dissociation energy, bead radius, bond length, etc.). A fundamental approach to the problem of modeling mechanically-induced polymer degradation is proposed. The polymer degradation is modeled on the basis of an integro-differential kinetic equation for the density of the statistical distribution of polymer molecules as a function of their molecular weight. The existence and uniqueness of the solution of the kinetic equation are established. The kinetic equation for polymer degradation is solved numerically. The effects of pressure, elongational strain rate, temperature, and lubricant viscosity on the process of lubricant degradation are considered. A comparison of a numerically calculated molecular weight distribution with an experimental one obtained in bench tests shows a good agreement. Two additional cases of lubricant degradation are studied numerically.