:Gearbox oil leakage is one of the largest warranty items for commercial vehicle manufacturers. The ASTM Oil Seal Task Force has identified two common seal failure mechanisms: 1. Seal lip hardening caused by a chemical interaction between the elastomer and lubricant 2. Seal lip deposits caused by high temperature lubricant/additive breakdown Significant improvement in seal performance can be expected by reducing the formation of these deposits through the use of thermally stable gear additives.This paper focuses on the aspect of seal lip hardening and loss of elasticity caused by interactions of the elastomer with the lubricant additives. Elastomers include: polyacrylate, ethylene/acrylic, fluoroelastomer, nitrile rubber and hydrogenated nitrile rubber. Statistical screening techniques were used to isolate the effect of aging time, temperature, and lubricant type on each elastomer.Laboratory data suggest that the fully saturated, peroxide cured hydrogenated nitrile rubber exhibits the best elasticity retention characteristics for a seal in contact with both thermally and non-thermally stable hypoid gear lubricants.