The Impact of Evolving Automatic Transmission Fluid Specifications on Base Oil Selection

Automatic transmission fluid (ATF) performance is determined by the choice of lubricant basestocks and additives used to formulate the fluid. The lubricant basestocks employed set the fundamental low temperature capabilities and resistance to oxidation of the fluid. Over the last decade, ATF specifications issued by the major North American transmission builders have required significant improvements in low temperature fluidity and oxidation stability. These required improvements have begun to limit the number of basestocks capable of producing suitable fluids. The practical impact of this evolution is that API Group I basestocks are rapidly becoming incapable of producing the new generation of ATFs. Recently issued, and proposed, specifications will clearly continue this trend. Future ATF formulations may well be forced to move to API Group II, Group III and/or synthetic base fluids to meet these increasing performance requirements.