Here is what I know.
1. The 6 liter did not come with AFM, so no worries there. Not sure where you are getting your info. The 5.3 and 6.2 did. The 4.8 did not. Take a look at your the top of your air filter box. It will state your engine displacement.
2. The AFM with the 5.3 had real problems with their AFM
3. The AFM problem went away in 2014 with the redesign of the Chev V-8’s. The 5.3 and 6.2 became Gen 5 engines with a higher pressure oil pump with an electronically controlled solenoid to allow 60psi pressures at highway speed and 40 psi at low speeds and idling. The AFM was completely redesigned. The 6 liter is still a Gen 4 and does not have the updated oil pump system. Nor does it have AFM.
4.There is an issue with the screen on any of the engines. The screen is right below the oil pressure sensor mounted at the rear driver’s side of the top of the block. If that screen plugs, the oil pressure sensor will read 0. I have no knowledge of any other screen other than at the oil pickup.
5.If you want your AFM system to remain healthy in a 2014 onward AFM equipped engine, you need to be running the GM recommended Dexos Oil. This is no place for old school xw40 oil.
6. However, running a xw40 weight in a high mileage 6 liter should not cause problems, in my opinion.