Ok lets think for a minute....would you put trans fluid in your car engine? or motor oil in the rear end gear case? :no: NO right]
Not necessessarily. I've used ATF in engines to clean 'em out. We used to put engine oil in the Road Ranger transmissions on our semis. Worked fine. Got in excess of 500,000 in a trans rebuild. A there are several autos & trucks now that use ATF in the manual transmissions, differentials, & transfer cases on 4X4s. Cat heavy equipment used hydraulic oil in transmissions & 50 wt motor oil in the diffs & final drives. 90 wt gear oil is almost a thing of the past.
NM