gear lube vs. tranny fluid

Well, I have used 80-90 in all my 833's for 30 years, My friend and his Hemi... it was a rebuilt 833, he thought he would go new school and try syn, clashed like crazy! He replaced the oil with 80-90.... perfect from then on. This on a suggestion from a good friend of ours who is lead mech at a Mopar dealership. Keep in mind I am just referring to 833 4 speed....not a fancy *** 5 or 6 speed.