as Scott says, but also try and clean the area with a hose pipe and your engine with a cloth so no oil marks so that you can see where its coming from, if its gearbox area is it where the driveshafts are coming from? as it could be the seals in the gearbox that are leaking, only about £3-6 to replace (each and there are 2) or it could be as mentioned, a breather pipe or rocker cover leaking down ontop of the gearbox, but clean it off and then you know what you are looking for.