To investigate the effect of aeration on fluid-elastomer compatibility, 4 types of elastomers were aged in three gear lubes. The four types of elastomers include a production fluorinated rubber (FKM) and production hydrogenated nitrile rubber (HNBR) mixed by the part fabricator, a standard low temperature flexible fluorinated rubber (FKM, ES-4) and a standard ethylene-acrylic copolymer (AEM, ES-7) mixed by SAE J2643 approved rubber mixer. The three gear lubes are Fluid a, Fluid b and Fluid c, where Fluid b is a modified Fluid with additional friction modifier, and Fluid c is friction modified chemistry from a different additive supplier.The aeration effect tests were performed at 125°C for 504 hours. The aerated fluid aging test was performed by introducing air into fluid aging tubes as described in General Motors Company Materials Specification GMW16445, Appendix B, side-by-side with a standard ASTM D471 test. Shore A hardness, tensile strength, elongation, modulus at 50% & 100% elongations and volume swell were measured before and after the designated fluid aging period (504 hours).The effect of aeration on fluid-elastomer compatibility showed a strong dependence on the types of fluids and types of elastomers. Aeration provides positive influence to fluorinated elastomers (FKM), but became detrimental to hydrogenated nitrile rubber (HNBR) and ethylene-acrylic copolymer (AEM). Among the three fluids, Fluid c appeared to be more aggressive towards HNBR and AEM regardless of whether the fluid was aerated or not. Both FKM materials showed good resistance in all three fluids.Aged fluid usually has a tendency toward oxidation, which in turn produces acidic by-products evidenced with an increase of Total Acid Number (TAN). Aeration, a process of introducing air to fluid during aging process, is expected to accelerate oxidation reaction, in turn generate more acidic by-products. However, testing results from this study actually showed a decrease of TAN of aerated fluids, even though aeration did cause catastrophic deterioration of the material properties for HNBR and AEM.