It would be best to almost fill it so that there is little room for the grease and it will be forced to re wipe. I agree with "too thin will leak". It must be thick enough to cling to parts. Thin will settle to bottom and not lubricate upper portions. After filling, cycle the box back and forth for a while, then fill thru hole some more. Tractors have much more turning usage than our trucks and more stress on steering components. They are "off road" all the time and turning at low speeds while pulling loads or dragging dirt. When I see oil higher than your gearbox on inner fender, I am slightly suspicious that oil is leaking down onto box from somewhere. Do I see that? At any rate if it's leaking like that, it's WAY too thin. Someone filled with oil. What I would do first is open fill plug, check for level AND what viscosity. Wipe box super clean, nudge the bolts that hold the covers on to be sure they are not too loose, drive and observe where leak it coming from. With old trucks, PO's can do anything, cardboard gasket, no gasket, thin oil...You don't say that it is hard to steer or wondering? "finally got back on the road" tells me it could be shrunk/degraded/dried out gaskets and thin oil combo.
