What fluid for a 4-speed trans?

How long do you want your transmission to last? From rebuilding A833s, in standard and O/D, I run 90 wt gear oil. The high wear areas, are the spines on the synchro hubs, thrust washers, cluster gear mating surface, and the countershaft. Not counting synchro teeth jammed and clashing from power shifting. This wear is much worse on a trans that was run with ATF. With 90 gear oil the wear is almost nonexistant. Will an A833 last 100,000 miles using ATF? Yes, but I want it to last as long as possible.