Need help with gear oil selection

I run Dextron II. Since the 70s.In all my A833s. Tried synthetic. Didnt work for me,too slipery.Syncros took forever. I modded my cones and blocker rings in an effort to make it work. Eventually switched back to ATF. The mods worked great for ATF too.I splash a little gear lube in some of them;depends how I feel. Maybe 20%. It is possible to make these A833s shift like lightning, even at 7000 plus rpm.Those gears are heavy.Cheers