I used 1 qt of ATF and filled the rest with 80w90. That seemed to give me the best feel when shifting. I tried all 80w90 and it was to hard to shift and straight ATF was just too loose feeling and gave me personally the wrong feel for shifting causing mis-shifts. So I compromised and mixed them.

Is there something out there better? Maybe, and yea, the Factory service manuals I have seen all called for straight ATF. But I will never run it straight in my stuff again. At the time too (about 10 years ago), ATF and 80w90 were CHEAP! Nowadays with dino oils approaching that of synthetics, I mght be inclined to try a synthetic when I get my 4-speed back together in a running vehicle.