Big5

I bought my current 2013 Sierra 5.3 ltr from an older gentlemen w/4K on the odometer in 2014. Truck now has 80K on it and has always averaged about 13.5 mpg. The best I've ever seen driving 60 mph on highway in traffic is just under 16 mpg. My truck has the code in the glove box which verify's it has AFM. I assume it must have been turned off prior to my getting the truck. Is it possible that the AFM was turned off? Also, the engine is burning about 1 1/2 quarts of oil every 5K. That's using mobile 1. Everything I've read on this forum says that's normal, but my last truck 2000 Sierra 5.3 burned very little oil even at 200K. I talked to my buddy that has a 2023 Sierra 5.3 ltr w/AFM and the best he has seen driving on the highway is 26 mpg. Night and day difference is MPG vs my truck.