I think I used 75W-90 when I replaced mine. That 80W-90 should be fine. On my 2003 trailer, there was one screw on the hub. I removed it then turned the assembly so the opening left by the screw was facing down and drained it. It takes a long time to drain.
Refill it kinda slow because it takes a little time for the oil to make it to the front where the viewglass is. I refilled mine to the same level they were at previously. Probably about 3/4 full.