Farm Force Bush Hog

So I recently picked up at 48" bush hog from a buddy of mine for a cheap price, it has low hrs and is in pretty mint condition, I have been using it for the past couple of days and it works great, I plan on grinding it down and painting it Kubota orange (it is yellow) but I was wondering if you guys knew what oil to put in the gear box for when I service it (I'm guessing 80w90) and also what rpm should I be running my BX25D, I have been running it at 2500rpm and it has been fine but I wanted to check.

BTW it is a Farm Force brand bush hog