gear lube vs. tranny fluid

I have always used 80-90 gear oil...Using ATF or Synthetics can cause gears to grind on down shifting because the syncro's are spinning in the low friction oils. 80-90 allows more "shock" on the gear security and some weight that slows down the syncro's allowing gears to engage instead of clashing. My buddy with his 67 Hemi 4spd GTX found this out after a re-build. He thought he would use what was recommended from stock.....went back to 80-90 and never a prob.