here is what tremec tech said:
Matt,
Thank you for your interest in the TREMEC brand product. I receive daily questions and comments on aftermarket transmission fluids. For every person that tells me that Product X is the best fluid to run in my transmissions, I get the same number of individuals that tell me Product X is the worst fluid you can put into my transmissions. I can only give you our recommendations for my product line. For the TR3650 the recommended fluid is DEXRON III / MERCON Spec ATF, (Texaco). Alternative fluids can be G.M. Synchromesh, G.M. part number 12345349, (semi-synthetic), or Mobil 1 ATF, (full synthetic). If you have any further questions, please feel free to contact me.