Eaton says they use conventional oil when they test/design the units.They state that using friction modifier, in any gear oil whether it be conventional or synthetic will reduce the amount of bias the helical gear set-up will have. In other words it won't lock as hard. They even say that different weights of gear oil have a subtle effect.

Friction modifier is used to reduce potential and actual clutch chatter in the clutch style units.

I am going to use a synthetic without a friction modifier added, which may or may not alter the bias. I have 3.91 gears with a .67 overdrive, so I am looking to reduce heat at modern freeway speeds with the synthetic.If it affects the bias too much, I'll drain the synthetic and go back to conventional gear oil. I just had a drain plug welded into my 8 3/4 to make gear oil changes easier.