Hello and welcome.
Is there any sign of leaks under the car?
How much oil does it use?
How much oil is it using?
OK.
I would take a good look under the car to be sure the undercarage is not full of oil.
Sometimes oil leaks can burnoff before hitting the ground, but you can still see a wet oilpan.
These engines are known for oil pan gaskets, rear main seal, and valve cover gaskets leaking oil.
If no leaks seen nder the car, then i would inspect the PCV valve.
If any work has been done to the intake manifold, a very common thing that people forget or overlook is to replace the PCV valve and its O' rings.
If an O' ring was left out, it would caus this engine to suck exteme amount of oil- which you have.
The PCV vlve should have an O' ring on the cover, and one on the under side.
Be sure there are 2 O'rings.
Again, on the underside of the cover, and one on the valve itself.
GM Dealer Technician
18 years plus at same GM dealer.