2 places I wouldn't recommend synthetics, a manual trans (synchros need a little friction, some people have problems because synthetics are TOO slippery) and a diff with a limited slip (again too slippery).
Brad (from the 4 Wheeling center of the universe, 4 corners USA)
If you want to find out what synthetic gear oil is appropriate for a given application ...... grab the manual and then go to Mobil's web site .... I'm pretty sure that it's www.mobil.com and go to the synthetic section .....all the lubes are listed with their "specs" .....that is, "meets" GM spec GL1409-AB3 and so on.
In a manual trans ....it's true that the blocker rings use friction (or at least pressure) to match speed (synchronize) with the repective gear that they're locking to the shaft ......... but I've only had trouble with standard gear lube in cold weather (a little grind here and there until the fluid "liquifies" a little more)....that wouldn't happen with synthetics. I did a complete change over to Mobil 1 motor oil in my Peugeot trans ...... since the manual said to use motor oil (this was a Peugeot trans in a Peugeot not a Jeep ..apparently gear oil is in short supply or very expensive in Europe). I specifically said "complete change over" because (just in case everyone doesn't know) if you mix gear oil and motor oil it foams on contact........anyway it worked fine in that trans without any problems.
So......did you get answers....or just more questions?