With a lot of miles on the engine it may be a bad seal, but something a lot of people don't realize is that you need to pull a vacuum on the crank case or it will find a way to release the pressure, and the rear main is usually the weakest point of release. Check your PCV valve, or better yet, install a new one and make sure you have it on a good strong vacuum source. I've corrected dripping rear mains with this method.