Your problem is related to "stryke1"(he posted the same thing). Anyway if the your rear main would be leaking you could see it either in the tranny inspection cover(auto tranny) or the bell housing where the tranny and the engine block meet(manual tranny). Of course, since these are both seals on the crankshaft (FRONT & REAR), it maybe leaking from both perhaps not, but I would recommend you to replace it together, you know like a love bird. This problem must addressed by a competent mechanic( wag sa tabi-tabi) as the crank may be off balance, due to larger internal problem.
Fix now not sooner.
check first if its coming from the rear of the engine (in between engine and transmission) if its engine oil then you need to pull down the transmission and remove the flywheel to replace the crankshaft oil seal.. you can have your clutch replaced if it is worn out together with the repair of your oil seal..