I entered the thread thinking you needed some help.
The 5.3L has a standard UIM gasket issue (similar to the 3.8L platform) and they should be swapped out. Hopefully there is a revised gasket (I have not yet done the research).
Another feature many 5.3'* seem to come with is what sounds like piston slap. GM has many tech bulletins on this and yet claims that it will not hurt the motor. In the little research I did it appears there are many that go 100K miles with a little slap and never have big issues from it. I learned a little something in a search....an obscure item that one will never forget. Check the exhaust manifold bolt heads if you think you have the beginnings of knock or slap. Missing a head or two? You don't actually have slap or knock, you have a leak. There are mixed feelings from mechanic friends as to the ability of getting all the bolts out w/o having to tear the heads off. Sooner or later this summer I'll try my hand at it while doing the UIM'* etc.
In the meantime...the 5.3L is one tough motor. I've over used and abused mine since I got it with 15 miles on the clock. Bent both driveshafts (w/o hitting anything) put more weight than ever intended on the truck and done 80mph down the highway etc etc. The motor has stood by and handled everything I've thrown at it with ease while getting better mpg than any other brand I know of.
I think I'll go hug it again..... I love that motor.