A bad valve seal would be the most likely culprit for oil leaking into a cylinder.
Either that or worn oil control rings.

 

You can perform a leak down test to determine if it's a bad valve stem seal or an oil control ring. Have you puilled the plugs to see which cylinder is burning the oil?

 

Agree 100%. Start by pulling plugs 007, to see where the problem cylinder is. Could be all of them, but you need a game plan going in. Plugs don’t lie.
Having said that, the PCV systems on these platforms let a LOT of oil into the intake. After you look at plugs, pull the air intake off the front of the throttle body, and shine a light into the manifold by pushing the TB open by hand. See if it’s got oil puddled up…

 

That's another good point.. If it is PCV related the oil can get pooled up at the back of the intake where there it can be sucked into the 7 and 8 intake runners then around the intake valves and into the combustion chambers.

 

The cats, if OP is running them, will mask some of the visual oil consumption signs. I had an 88 IROC (yes, SBC, but engines are very similar) that I just took for a half hour drive to fully warm the engine. I had to leave it idling for 15-20 minutes due to my father's failing health. Car was not in the garage. When I returned, blue smoke was coming out of both tail pipes. Within the next 2 weeks, my cat was virtually plugged, and car wouldn't go over 35mph!! It was valve guide seals, which were just O-rings on the old SBC. My guess is guide seals....