My truck is a 2008 GMC sierra with 5.3 and AFM. Did not have an issue until I swapped oils that I was using. I have been using royal purple and swapped to Mobil one because it was a little cheaper. After a little whiule I notice a little puff of smoke on cold start up. Now I have about 25% left before next oil change and noticed the lifter tapping after gassing it going down the road. Also noticed going down the road the oil pressure would bottomed out and jump to 40 psi. That is when I noticed it was low on oil.
My truck was tuned with about 14000 miles on it and AFM turned off. Now it has almost 33000 miles on it. I think it is the oil but it could be coincident.