The spec calls fro GL-1 oil( same as the transhydraulic oil use in the tranny/rearaxle/hydraulics on the tractor. The fill is 2 quarts and the level, per the TO-30 Owners Manual is "...at the center of the steering arms." I think tht means at the center line of the transverse part of the steering shafts that come out of the sector gears.
Some people fill it with corn head grease when the seals are shot and it won't hold fluid. That's a temporary fix until you can fix it correctly. It's really not designed for a grease pack.