What fluid for a 4-speed trans?

I have an A833OD trans going back into my truck. I drained it a while ago and now I can't remember what it was filled with. The FSM says to use ATF, yet somehow I recall being told 80 weight gear oil was a better way to go? And of course on my truck parts shelf I have both ATF and gear oil standing by so there is no guidance to be had according to what's on hand.