"The seal on the Prop Shaft is
"The seal on the Prop Shaft is certainly a likely cause. However, before commencing any work, I suggest you consider the following: Sometimes engine exhaust and two stroke oil residue dripping down from the exhaust stack inside the column, can cause you to believe the gearbox seal is leaking.
If the leaking oil is very dark in colour it is highly probable that it is exhaust residue.As a double check, loosen the lower gearbox drain screw, the oil that drips from there will be either a nice translucent looking colour or a whiteish opaque colour.
If it is whiteish, it is indicating water has entered the gearbox through the worn seal and the internal gears turning at high speed have emulsified the oil with the water. If thats the case, you will need to have the seal replaced and the gearbox thoroughly flushed prior being refilled with fresh oil."