No synthetic......run just regular trans fluid. The syn will cause the clutches to slip some and in return build heat. Yes the syn fluid will take the heat better but the clutches don't like it. The regular trans fluid will allow the clutches to grab or lock up better than the syn..........in return shift harder. I did my own experiment with my truck, ran syn trans fluid in it when it was built, then slowly drained out the syn fluid overtime by pulling the plug and I could tell a difference in how it shifted overtime as the syn fluid disappeared. The truck always broke the tires loose in second but now it keeps the tires broke loose longer....?????.......and the third shift was definetly harder than before.
This is just what I experienced.........
Hope this helps.

James

 

Mobil1 in the Allison since day one for me. Mobil1 in the crank case as well as the diff's too.

 

Bump, anyone else? Guys running FLT's what are you running? Just any old Dex III?

 

Who all removed the check ball in the end of there input shaft for firmer TQ lock up?

 

If you have an FLT trans they remove the check ball...so there shouldn't be one to remove. I use ac delco dexron III.

 

I dont have a FLT trans, I was just trying to decide whether to remove the ball or not. I imagine I will.

 

I am using good old Dextron III in my FLT as recommended by the FLT guys.

 

Cool, trans is coming the rest of the way apart shortly.

 

I Don't run synthetic on my FLT.

 

I'm running just regular DexronIII. Seems most people recommend using reg oil in these trans