blue smoke from exhaust from 96 Riviera

After 20 to 30 minutes of driving I see blue smoke from exhaust, which indicates burning of oil. PCV valve has been changed. Can anyone tell me the cause of this? Thanks!!!

The usual cause is worn valve guides/seals and/or worn rings. If there is too much oil up in the heads, it can make the car smoke. Years ago when I was young and inexperienced, I replaced the valve cover gaskets on my 1972 Skylark 350. I scraped off the old cork gaskets, and missed a few big pieces when cleaning out the heads. The result was blocked drain back holes. Oil was sucked down the guides and the exhaust would smoke blue.

Wow I never knew that. I was under the impression that the plastic lower intake gaskets are shot and I bought the aluminum ones. They haven't been replaced before and the car has 208,000 mi. I'm thinking about just overhauling the entire engine.