A fundamental approach to the problem of modeling mechanically induced polymer degradation is proposed. The polymer degradation is modeled by a kinetic equation for the density of the statistical distribution of linear polymer molecules as a function of their molecular weight. The integrodifferential kinetic equation is solved numerically. A comparison of numerically calculated molecular weight distributions and lubricant viscosity loss caused by polymer degradation with experimental ones obtained in bench tests showed that they are in excellent agreement. The effects of pressure, shear, temperature, and lubricant viscosity on lubricant degradation are considered. The increase of pressure promotes fast degradation while the increase of temperature depending on other parameters may delay or promote degradation. In some cases, the density of the molecular weight distribution function maintained its initial single-modal shape and in other cases it changed with time from a single-modal shape to a multi-modal one.