Most likely you have what is known as blow-by. That happens when the piston rings no longer control combustion pressure as well as it should, and some of the combustion gases blow past the rings and pressurize the crankcase.
This is usually caused by either sticking rings or by excessive ring and clyinder wear. The cure is usually to overhaul the engine.
Replacing the PCV valve was good advice. You should also make sure that the rest of the PCV system isn't plugged up. You should also check the condition of the oil (it sounds like you have).
If the rings are carboned up and sticking, sometimes you can get them functioning again with chemicals. The same stuff that is used to clean up the intake manifold will sometime help if it isn't too bad. Removing the spark plugs and squirting about a tablespoon full of something that will break up carbon will sometimes help (certain types of penetrating oil, or even auto trans fluid).The guys at
www.bobistheoilguy.com have been touting an oil additive called Auto RX that cleans up sludge claiming that it can clean up ring packs.
It may be worth trying some of these things, but don't get your hopes up too high.
