Base Stock Developments in Automatic Transmission Fluid Design

Automatic transmission design has undergone significant changes over the past two decades as automotive companies pursue improved reliability and ease of operation while addressing mandated legislation by government and regulatory bodies, particularly in the areas of energy efficiency and emission reduction. This has also impacted the design parameters for the fluids that are used in these new transmissions, including both the additive chemistry and the lubricant base stocks. The selection of base stocks has transitioned over the years from wax-free naphthenics to solvent processed paraffinics, and more recently to hydroprocessed and catalytically dewaxed paraffinics. These highly saturated and high performance base stocks have several unique features that make them ideal for present and future automatic transmissions, including high Viscosity Index, low n-paraffin residual wax content, low volatility and excellent response to additive chemistry.

The effects of high performance base stocks on automatic transmission fluid design and performance will be discussed. This will include base stock effects on both high and low temperature fluidity, oxidation performance and other important transmission fluid properties. The use of very low viscosity fluids will also be discussed as they can provide some interesting options to the lube formulator in terms of fluidity modification or dumbbell blending.