Its the DoD system, "Displacement on Demand" its where the computer shuts off 4 cylinders to get better fuel economy on the freeway.

It is very common for the 5.3 motor with DoD to burn 1-2 quarts per oil change (3,000 miles). The oil seeps pasts the rings on the piston and gets burned up. You can try replacing the PCV valve to maybe slow it down some, but General Motors says its a normal thing to happen.