Re: 67 Johnson leaking oil from shift linkage cover
The shift rod connector access plate leads to the exhaust chamber. There is exhaust in there. The oil that you put in the gas goes through the engine where some is burned and the rest goes out with the exhaust, where it picks up carbon particles and turns black. That is what you are seeing leaking out the cover. It is totally harmless and was heading to be discharged out the bottom with the exhaust anyway. If the mess bothers you, cut a gasket and put it behind the access plate.
That are is WAY above the gearcase and there is no way gear oil is leaking out way up there.
EDIT: Several reasons lead to wet, black plugs.
1. Carburetor adjusted too rich, sending more fuel to the cylinders than it can burn
2. Running at prolonged idle, not developing enough heat to burn the stuff off the plugs
3. Ruptured fuel pump diaphragm
4. Running it in a bucket or barrel.
5. Some wetness/oilyness is normal
Especially #1