I have an old Bush Hog 13' flat rotary cutter. The outboard seal was leaking BAD on mine so I replaced it. The hardest part was getting the stump jumper off. We ended up cutting a hole in the deck and sliding a large solid bar down thru the hole to the topside of the stump jumper and then pounding on the bar with a 16 pound sledge, we also applied heat. Wow, it was a tough bird to remove.
Once it was off, piece of cake to replace the seal. And the seal was only around 10 bucks if I recall.
As far as quantity of oil, the two out board hubs on mine have a 1/4" "fill to this level plug" that when oil runs out - it is full. The center hub just has a fill opening part way up on the gearbox, and you put oil in it until it starts to run out the same hole. Pretty simple. I use 90wt gear oil, but then my seals are in pretty decent shape.
Ditto what has been said re: checking the bearings and the grease if your seals are leakers.