I have a K model 977, if you have a late model H it might be the same; it may also be hidden behind a lot of mud and crap. look in the underneath center rear of the bevel gearbox(very back end housing of the machine)-the bevel gear drain plug is right in the middle,I think it was 3/4"square drive plug,(try a 3/4"ratchet with extension)..if you're gonna pop that plug get ready for a tidal wave because its like 40gallons of crude comin at ya if she's full.
As said by Cat977, I'd use kerosene as well,it cuts crud pretty well..hard to find it this winter, something I heard that they stopped production of it for some reason.Expensive at the least.Best of luck