For the Hilliard front differential its the Polaris Demand Drive fluid. For the internals of that diff...I've stuck with the Polaris lube for 10 years because I don't really know what the critical aspects are for that lube. So why mess other lubes?
In the RZR 800 I started running Valvoline 75-W90 synthetic gear lube. I got that from Hawk Engineering when they did the "fix" on the pinion shaft and the CNC cases. Worked just excellent. I began running in the transmission about 2013. Never had a problem and using a laser temp gauge, my diff ran 10 degrees or so cooler than a friends with OEM AGL lube.
The Valvoline is less expensive...and you can get it just about everywhere!
I purchased the 5 year warranty from Polaris, so I'll probably stick with the OEM lubes until it expires. After that, well....
Just thoughts,
Pirate