I agree that in general replacing the gasket or seal is the best policy. However, I've been convinced that ester based cleaning products can, perhaps 60% of the time fix rotating seal leaks. I would probably degrease the enigine some and look for the leak source, or even use the UV dye...but that's just me. I would also consider snugging up all of the valve cover and oil pan bolts if accessable. You didn't say how much it leaks?
I am more familar with AutoRX than the other similar products on the market, and they also happen to stand by their product pretty well. On the downside, I think it runs something like $20 a bottle (that's if you get the discount for being a member of an internet oil discussion forum, which I think this forum should count).
Oh, if it turns out that you had a leaking pan or valve cover gasket, the AutoRX will have only improved the internal condition of your engine in the process.