Most rear main seal leaks are caused by a combination of normal wear in the crankshaft & seal and the drying, hardening and shrinking of the main seal. Bar?s Leaks Concentrated Rear Main Seal Repair restores the seal to like new condition and also contains additives to stop leaks in severely worn seals and crankshafts. Works with ALL gasoline and diesel engines using conventional, high mileage and synthetic oil. Specifically designed to seal rear main leaks, but also works better than a conventional stop leak on all other leaks. This includes the timing cover seal, cam seals, O-Rings and other gaskets.