If it is like the Mobil 1 ATF and the Allison Transynd (Castrol) then it has a better coefficient of thermal transmissivity, meaning it transfers heat better. If the transmission ever gets to the breakdown temperature of even standard ATF, ... there's already something wrong.

The syn fluid is better, whether you'll see measurably better mileage or transmission life is another story. From new? I always go syn. The cold properties are what I really like in synthetics as it gets cold here in the winter, fluid that is the proper viscosity cold is always better than non-syn peanut butter fluids IMO.

Swapping ATF brands around is something that I avoid when possible (difficult to know what's in a used car though) as different seal swell properties can permanently damage seals and cause internal as well as external leaks, ... just a caveat.

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Gone to the dark side

- Jeff