[2009Toyotoad]

My 02' 8.1 liter Silverado 3500 consumed half a quart of oil every 1,000 miles until it reached 60k+ on the odometer, then it stopped using any excessive amounts of oil between changes. My wife's 2010 Avalanche 5.3 liter never uses any oil and she's at 30k+ miles. There a long history with GM V-8's and oil consumption.
My advice be persistent but patient with the dealer. This issue is likely to go several rounds before you get service.

One other thought, it may not be the AFM system causing the issue. Chevy/GM has continue to use those really hard piston rings for several decades. Those rings have a very long break in period to properly seat in the cylinder walls. Try running to 10% or less on the oil life monitor before changing oil, especially since you are adding 2 to 3 quarts during the oil life cycle. If you continue to have AFM issues get the shield checked out. It could be poorly installed or sealed. However, I've noted my wife's Avalanche not moving to v-4 when I expected it too, and she's not having any oil usage problems.