Hey toy....
Good question.
Answer: Automatic Transmission Fluid is spec'd for several manual transmissions - Ford, Mazda Mitsubishi, etc.
How good is ATF? Some auto manufacturers don't recommend changing fluid before 100,000 miles!!
ATF has a viscosity of about 5w-20. I could see someone running ATF in a manual tranny in a rockcrawler where the climate is extremely cold.
ATF is formulated for lubrication, but also for heat absorption. Manual transmissions don't generate as much heat as automatic trannys.
ATF acts as a good detergent. If you picked up an older manual tranny and wanted "clean" the inside, you could fill with ATF, drive it for 100 miles, and drain it and see how much residue or metal was in the transmission.
In my early 1982 Mercedes diesel, I used to put in a quart of ATF in a full tank of fuel about every 4 fill ups for many years. It cleaned the cylinder and combustion chambers really well.
While I was breaking in my newly rebuild W-56 I used Castrol 10-40 motor oil, recommended by the shop that rebuilt it. Then filled with Redline MTL.
In my VW transaxle on my 3 sandrails, I used 20-50 motor oil, rather than gear oil.
I'd be using whatever is spec'd for that transmission and running Redline or Mobil 1 Synthetic. I would not hesitate to install that tranny unless there are known quality issues.
Gnarls.
