Instructions how to replace gearbox seal on 1018 john deere bushog

You replace it.

Likely a failed shaft seal.

From the lt155 manual;
The transmission is a sealed component. No maintenance is required on this transmission. If you suspect any transmission problems, please contact your John Deere dealer.

excessive oil put in
oil cap left loose
oil drain plug loose

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
Best Manuals

I am not sure what you are calling gunk but if this gunk is black it is engine oil. The tractor and the engine was built by Yanmar. The injection pump is lubricated with oil pressure from the engine. The seals in the pump will leak and engine oil will get in the fuel system and contaminate the diesel fuel and plug the fuel. Seals will have to be replaced in the injection pump to correct the gunk.

A seal may out in fuel pump

you can fill the gear boxes with grease but also have some oil in there to so they lube till they get warm and grease breaks down reason the seal wont hold is that the seal has wore into shaft

sorry but its going to be a big job. you need to remove the gearbox to open up and get to the seals. you could try and use an additive in the gear box, its like oil seal for engine you can get it for engines gear boxes and difs and power steering. it sometimes goes by seek and seal, in older vehicles its quite good. you will need to replace the oil it will need to be done twice to clean it all out so it can get a good seal.. i.e change oil run engine get the gearbox to move for 20 min then change the oil as its warm not hot so let it cool down for 10-20 min then drain. put the seek and seal in and top up with oil and run as normal, check after a few days to see if its worked. usual does.

To check trans/diff oil, depending on how old the unit is, a good many of those type are sealed units and you cannot check or add oil to them. Again this depends on the year and model. Check with a John Deere dealer and they'll be able to tell you for sure. Matt