I am changing the front and rear gearcase fluids in my 03 Sportsman 500, the owners manual says I can use a GL5 80-90 weight gear lube. Mobil makes a synthetic gear lube but I could only find 75-90 weight. Castrol makes 80-90, but it is a synthetic blend. Does anybody use either of these?

 

Nope.
I use Polaris fluid in all the cases. Only the engine gets other than Polaris. This is a recommendation from other experts.

 

what do you use for motor oil?

 

Good advice.

 

I went with the Mobil 1 0W-40 Synthetic for the engine.