What colour is it?
If it's black then it's engine oil, it could be leaking from the back of the rocker cover and running down but if it's coming from the housing it will probably be the crank seal behind the flywheel.
If it's gearbox oil - probably red in colour (look closely it might look like engine oil) it will be thinner and smell differently then it's gearbox oil or green if someone put EP in it, it will stink.