That part about it doing nothing for the valve cover gasket on the same truck makes total sense to me, because AT-205 is a seal softener and it only works on rubber stuff that's dried out and shrunk. I had a similar deal with my old Jeep, the rear main was dripping pretty good and a buddy told me to try that stuff, and sure enough it slowed way down after about a week. But the oil pan gasket right above it kept weeping the whole time, so it's got to be about what kind of seal it is and whether it's actually rubber that can swell back up. A valve cover gasket on a 5.3 is probably just too far gone or it's leaking from somewhere else entirely, like a cracked grommet or loose bolt. The stuff can't do anything for a gasket that's physically torn or for a leak that's not even coming from a rubber seal.