On the gear box, is there a small square plug and a large one, the small one when removed (if so on this model) would indicate the correct level of oil, some should spill out, if not top off, flip over unit, drain, change oil. I use Lucas 85W-140, or 90 wt. in my rhino, most I have seen use similar, my book called for 00 or 0 type lube, which is thicker, stringy/tacky, probably does not leak beyond seals, mine seems to not leak, seals holding. I could not find the 00 or 0 in small quantities, the Lucas works well and stays clean a good while, I do change mine annually, some never bother, each to his own.
One of my friends has a small 4'-0" model, not sure what kind, it has both plugs but the bottom one has a grease fitting, which probably was installed long after it left where it was built, or gearbox built. Some with leaky seals use cartridge grease when shaft seals leak.
Personally, I'd rather fix the seals, because who knows how many different kinds of grease, how much is in there, venting, and or mixing non compatible soap bases (different grease's mixed). What if you want to change or clean out the gearbox lube, potentially a mess, or maybe not ??? There may also be a vent in the filler plug, one of ours has that, let that get plugged and it pushes lube out somewhere. I've run these kinds of mowers since I was 10 years old, never had a gearbox failure. Many others use regular grease in them, I've always wondered if that lubes properly and or long term effects, as those gear boxes are kind of like differentials, all have an oil or something more viscous than grease when new, I believe, good for discussion, and also good to mention that I'm not familiar with IH models, but never let one run dry that is for sure.