I have a '07 Classic 5.3, motor was swapped a few years ago and only has 50k on the motor. I went to change the spark plugs and noticed there ...

I have a '07 Classic 5.3, motor was swapped a few years ago and only has 50k on the motor. I went to change the spark plugs and noticed there was oil on cylinder 2, 4, 6, and 8 plugs. And I can see oil sitting where the plugs go into the heads.

I was reading it could possibly be as simple as replacing the valve cover gasket so I went ahold and got those. But I have yet to do them. I know they are simple to replace but I was wanting to get another opinion.

I understand there was a revision to the valve cover for the 07-11 or 13 nbs, but wasn't sure if there was a revision to the 99-06 body style.

That is the usual culprit. Make sure the flange on the valve cover is straight before bolting them back on.

Yes, swap out the gasket. And the revised driver's side valve cover only applies to '07.5-'13 motors.