the manual should say what is/was in it from the factory,however if you're not the original owner it may have been changed.....
if you're in Alaska (really cold) ,you need a 'thin' oil that'll floooow when it's bone chilling cold.
That’s because ALL vehicle rear-axle/differentials fail so regularly in Alaska because they use gear oil… right?
One way to determine what is in there is to drain a little out for a sample. Gear oil will usually have a strong “garlic” smell and appear quite dark, while SUDT is amber after it’s been used awhile. My manual states it came from the factory with SUDT and to replace it with either that OR gear oil.
When I drained the front axle I only got about a quart out of each side, and I re-filled it with 80/90 gear oil… 8.5 qts per the capacity spec. (Yes, it was clearly under-serviced)
That’s when the left bevel-case oil seal was discovered to be leaking. (Perhaps the axle had leaked-out the original clear SUDT and it went unnoticed if the tractor was stored on the ground by previous owner..who knows… Anyway, it may have once-been properly serviced and my adding gear oil made it obvious over-night that the oil seal leaked.)
I replaced the seal (wrote about it elsewhere in these forums) and serviced the axle again with 8 qts of gear oil. It and me are both happy.