I had a customer who I furnish a new , from Cass , clutch type 8 3/4 posi unit that he had a local shop assemble and installed. I furnish 2 quarts of the non synthetic Valvoline 75/90 WT gear oil along with a bottle of friction additive for the clutches, the shop guy who installed it in the Lawyers car(my customer) use their synthetic gear oil with no modifier.
The owner called me very upset that he had almost wreck his 1967 Plymouth GTX on the way home foromm the shop when he started to turn right up his road to his home and the posi unit locked up and almost made him go straight forward into the ditch instead of letting his car go around the corner like normal.
I replaced the unit and he had the same experience again so I ended up providing him a helical gear type posi before I found out what that shop was doing to make those posi units act that way
My message is to make SURE you use the correct gear lube for your type unit