Dextron III / IV or Synthetic ATF

Originally Posted by jerry witzel

Dexron HP fluid, which is what GM is now using in the 8 speeds is fully synthetic. Some of the the 8 speeds that were filled with Dex6 are having tcc shudder issues and they are having us flush and refill with the HP fluid. I fixed a very low mileage 17 Canyon with this procedure. The GM part number is 19300536. Mobil 1 also has a version. Stuff is pretty pricey though.

Mobile 1 also has Allison approved Mobile Delvac Synthetic Trans fluid TES-295 #112810. Better blending stocks than Castrol.
I agree with Ed, your trans builder knows...