The best thing to do is clean the area thoroughly with degreaser, then when the leak reappears try & track it back to the source. This could be the rear seal of the engine, the front seal of the gearbox, or even just a leaky rocker cover. I had this last one & the oil was on both sides of the motor all the way back to the rear of the gearbox. It was a real puzzle til I did what I suggested, then it was obvious the rocker cover gasket had gone hard & shrunk. Good luck, I hope it's just a simple leak, because replacing a rear seal means taking the flywheel off, which means etc. They are a pain in the rear seal .