So I bought a new to me 04 sportsman 600 a few months ago. I tried to take it out on the trail the other day, and found the AWD was not working. I checked out all the electrical components, which seemed to be working as they should, but still no AWD. I put the ATV up on blocks then I drained what little oil there was in the front diff (about half a cup) and added some 80W90 gear oil. After revving it up for a few minutes in gear the front wheels finally started working.

So my question is what kind of oil is supposed to be in the front diff?

Doesn't use 80/90 in this differential! Uses Polaris Demand Drive Plus hub fluid as the awd is in the diff and not in the hubs as previous models. Item #9 in the parts list is the weak link as they can crack. Most replace the platic roll cage with aluminum.Dirt Cheap Yamaha, Polaris, Suzuki & Kawasaki OEM Parts & Accessories – Cycle Parts Warehouse http://www.ebay.com/itm/Polaris-Sportsman-600-700-ALUMINUM-Front-Differential-Roll-Cage-3234167-3234455-/161650664345?hash=item25a3216399&vxp=mtr