Either is fine. I'd stick with a conventional no matter what which helps the gears run cooler. Even Revolution Gear specifies a conventional gear lube. 80W-90 is fine, 85W-140 is fine for heavier uses. I use an 85W-140 conventional gear lube in my Rubicon.
The one time I used a synthetic a gear-guru guy looked at my gears and accused me of running a synthetic from seeing that they had gotten hot. He was right, I switched back to a conventional. Even Currie and Dynatra require conventional gear lubes in their axle to keep the warranty intact.