This actually happened because Synthetic oil, while providing FAR superior protection, is much thinner than conventional oil. You may even notice a slight drop in oil pressure. Putting conventional oil back in may stop the rear main from leaking for now, but it is on it's way out, you will be in there soon enough replacing it. :roll: If it is the two piece seal it really isn't that bad of a job to do. Worst part is taking off the oilpan.