I apologize for reviving this thread!
I understand that many here would disagree with me, but I have had great success with a couple of popular over-the-counter stop leak products. I would not mention the name of the product, but it is a very popular brand you could get from virtually anywhere, from Autozone to your local grocery stores for around %$7.
Anyway, it was on a friend's 1990 Honda Accord daily driver with a leaky rear main seal. The car had about 160,000 miles on it when we found out that it was having a severe rear main seal leak. He wanted get rid of the car because of the leak. We figured that replacing the rear main seal would not be worth it for a car like this. He'd rather buy another daily driver than spend any more time repairing the car.
After pouring the stop leak product into the crankcase, we cleaned up and detailed the underside and the engine bay of the car to enable us to inspect for leaks. The leak gradually stopped after a few days. We let the stop leak product work in the engine for roughly 2000 miles. After that, we never poured any stop leak product into the engine. Between oil changes, there is a very very light film of oil around the transmission bellhousing, but no oil dripping whatsoever like before. It's been around a year and the rear main seal leak hasn't come back yet.
Truthfully, like most members here, I am not a fan of using additives to a perfectly running engine. Normally, I would get dirty and repair leaks from places such as valve covers and oil pan. Valve cover and oil pan gaskets are very easy to replace. However rear main seal is extremely time consuming to replace. If the car was a manual transmission car, we would replace the seal while we change out the clutch. However this car is equipped with an automatic transmission and we really don't feel like dropping the transmission for the task.
The 10 hours or so time we would spend on repairing the rear main seal on a 20 year old Accord is worth more than the car itself. Nevertheless, for around $7, my friend and I are very satisfied. It effectively turned a dripping leak into a leak that only develops a very light oil film around the bell housing.
I am no oil expert, but I notice that many high mileage oils on the market claim to contain additives that "swell" rubber seals to prevent oil leaks. To me that is what most leak products do: "swell" rubber seals with a higher concentration of this additive.
Anyway, I personally experienced no harm to the car when using a stop leak product for only 2000 miles or so. Even if a product doesn't work for you, you can always drain it on your next oil change.