5.7 has the sniffles

The Good thing about these engines it they are all O-ring, and fairly easy to work on.
The bad thing is all the factory stuff is TTY.

If it were me, i'd pull that head, and see where i was before wailing on it, or putting more mystery goop in it. Sometimes things like the lucas oil additive can do some "cool" things like re-pop lifters and such...but sometimes you just gotta see what's going on. The engine bay is pretty open on the LX. for the price of a head gasket and a set of tty head bolts, i'd probably pull the head while it's a "head" problem. Before it becomes a " dropped a valve and busted a piston,which then sent pencil shavings through the entire bottom end" problem.

If you go this route, and a local dealer wants an arm and leg for the parts let me know, I can get you an OE head gasket and head bolts pretty reasonable.