Rear main seal leak

I have a rear main seal leak on one of my cars. Is there something that I can add to my oil to swell the seal. I think I posted this question before for my 71 Super Bee, but I can't remember the answer.

If the leak is minor, you could try a high mileage oil.

The car has about 56,000 miles. We purchased it last year, and the previous owner used regular motor oil. Since then I've used synthetic oil.