What kind of gear oil for 8 3/4? Why not synthetic gear oil?

I did some searching online and this is what I found from tractech the maker's of the detroit truetrac (an eaton brand)

Unlike the Truetrac, most limited-slip differentials like the one shown above operate like open differentials but have spring-loaded clutch packs to mitigate the otherwise unrestricted slip between the two sides of the differential. This is a far inferior design to that of the Truetrac. These clutch packs wear out and the differential becomes more and more like an open differential with use. Also, these differentials create handling problems on high-traction surfaces like the road because they want to remain tight and prevent differentiation between the two wheels. Finally, these clutch-driven limited slips require special friction modifiers to be mixed into the gear oil to permit the clutch packs to operate smoothly.

Tractech Truetrac Differential

The Truetrac is gear-driven, just like the standard differential, and operates on a completely different principle. The Truetrac operates like an open differential in high-traction conditions. On low-traction surfaces, the helical side gears prevent one side from spinning out of control while the other side remains stationary, as often occurs with an open differential. During times of severe differentiation when one side of the axle wants to spin out of control, the helical worm gears are pushed away from the larger helical gears and pushed into the carrier. The additional resistance on the side with less traction diverts torque to the other side of the axle. The gear-driven operation provides several dramatic advantages over the clutch-operated designs. There are no clutch packs to wear out on the Truetrac, and a Truetrac wears no faster than any standard differential. The operation of the Truetrac is also smoother and more transparent than the clutch-operated designs. Finally, there is no special friction modifier required for proper use, and standard 90wt gear oil is the fluid of choice with the Truetrac.

I also found a bunch of offroad sites that love the truetrac and they all suggest synthetic gear oil.

My driveline guy suggested 85W140 GL5 non-synthetic. Everything else I've looked at seems to differ. I think I'll just run the synthetic for now and check it after 500 miles. I still have time to decide but it makes since that I could use synthetic since their is no need for friction modifiers. The Redline stuff does come with friction modifiers already in it though. The debate goes on I guess.