How do I know if Rear is still good?

So I believe this is what I got, 8 1/4 rear end (by counting the bolts on the Cover) 3.55 by seeing the tag on the cover, and sure grip by putting the rear on jack stands and spinning the wheels and seeing them both spin.

I'm draining the fluid now, it seemed good on the inside, no metal shavings it anything, a little tiny bit if rust on some of the teeth but only what was not submerged in oil I think do to the pressure wash a tiny bit of water got in, scotch brite removed the rust so it's all good now.

What type of fluid should I put in this rear? How much? And where? Also I didn't notice any gasket on the diff cover... Is there some type of gasket for it? Thanks