The earlier models came with a drain plug and a fill plug in the trans-axle. Later models included a "level plug" because they changed the amount of oil Case put in the trans-axles. No one has uncovered the reason why this was done. On your tractor, you should operate the tractor until all the fluids are nice and warm and then unscrew the drain plug in the trans-axle and leave it drain until all you get is the occasional drip. Then put the drain plug back in and pour new oil slowly into the fill plug hole until it begins to flow back out. When it stops flowing, install the fill plug. You are done. As for oil, you have several choices. You can use common hypoid oil that is used in cars and trucks, although there is no real reason to do so.. OR...you can use 20W50 or 15W40 motor oil or 30 weight motor oil. All will work equally well.
As for the hydraulic system, 15W40 is the popular choice for year around service. In very hot climates, 20W50 is a better choice. In cold weather 10W30 will help in engine starting.