GM-GMC: 2001 gmc 2500 hd sputters when under load, cylinder lubricant, poor mileage

Question
I was getting poor mileage so I replaced the plugs,air filter, and ran a bottle of Lucas upper cylinder lubricant/Injector cleaner to improve this. It put me from 10mpg to 12mpg for about 200 miles now under acceleration it misses or lugs. So I figured a plug wire went bad from removing them and put a new set of plug wires in and same thing. When revved in nuetral it doesn't stutter at all, only under load. I am thinking fuel filter but thought I would ask first, maybe the Injector cleaner cleaned gunk from tank and plugged something? To date the check engine soon flashed on breifly once but didn't stay lit.

Answer
use some sea foam in the crank case and gas.

mine did this and it was the knock sensors that were going bad but the engine light came on.
i wouldnt suspect fuel issues it has to do with the power of the engine and it maybe gunked up so use some of that sea foam and see if that helps