Factory is standard PS fluid. Redhead and apparently Borgeson use ATF to break in the boxes. (Modern pumps use ATF). I called Redhead when I got mine and asked what to run and they said "we use ATF but either works." On other threads/forums, it is based upon the pump's requirements. Since our pumps use ps fluid, that's what is recommended. I installed mine with ATF to keep the Redhead happy under warranty but the pump's been leaking forever (bad pressure line seal) and is noisy. I plan to flush it and replace the pump again and use PS fluid this time.

 

 

I run ATF in mine never had a leak with 2 Gear boxes and factory o-rings