Late comments: Yes, the typical "fix" for leaking mainshaft seal is to put in grease instead of gear oil. Yes, many dealers recommend cornhead grease. The reason is that cornhead grease is formulated to cling to the gears. But BY FAR the most common practice is to use "00 Grease." Your friendly Tractor Supply sells "double zero grease" in a squeeze bottle the same shape and size as 90 wt gear oil. I hear it is used all over the south for bush hogs and has been for years. It is a heavy lube somewhere between gear oil and much closer to grease gun grease. Not quite the same as "cornhead grease" but easier to put in using the squeeze bottle. In my view the "00 grease" is fine and unless the seal is so bad that it won't stay in the gearbox you'll be fine with it.