Use the Correct ATF for Your Transmission

We have all entered a debate at one time or another about which ATF to use. With so many different preferred fluids for the various makes and models, why not use a universal fluid? What sets all these different fluids apart? Why so many and what is the difference?

​​​​​​​Much like motor oil, the differences are in the viscosity and additives.
Motor oil is designed to withstand combustion events, clean carbon from the engine and prevent friction. While transmission fluid has some similarities to motor oil, it can contain additives to aid in friction with wet clutches.
The other big difference is the viscosity of the fluids. Dexron® III is roughly 4.2 centistokes (cSt), while the new ultra-low viscosity (ULV) ATF is 6.1 cSt with a slightly different recipe for mitigating friction between hard parts. I have heard a lot of stories about rebuilders changing the ULV ATF out of their 10-speed transmissions with regular Dexron VI and experiencing failures shortly after. Everyone wants to know why these failures occur.

The transmission type will dictate the appropriate ATF. Modern 8-, 9- and 10-speed transmissions have parts moving at much higher speeds than older transmissions. This requires a different recipe of friction additives to be added to this ULV fluid. The proper viscosity of the fluid is key to keeping the flow rates of variable force solenoids (VFS) in their operating window. VFSs control pressure based on signal/voltage from the TCM/PCM and can control clutch apply pressure, line pressure, etc. Other solenoids, like linear or EDS/PWM, are less likely to be affected by viscosity.

Orifices in the separator plate play a critical role in clutch control valve timing. When fluid flows through an orifice, it gains velocity but loses pressure.
A higher-viscosity fluid has greater resistance to flow, providing a low flow rate and increased pressure when fed through an orifice. This would be opposite for a low-viscosity fluid.

Transmission temperature will affect the viscosity of the fluid as well. Pushing a transmission past its temperature threshold will cause this fluid to break down and lose viscosity, which will often cause the fluid to darken and smell burnt. This is bad because, unlike in an engine, there is no combustion event. The black material in the fluid is friction material, and the only place it can come from is your clutch packs or converter lining.

At the end of the day, don’t gamble or cut corners with ATF. Use the factory-specified fluid to keep the vehicle going down the road.

Caleb Perham is a Sonnax automotive technical specialist. He is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Transmission Company technicians.