I have been running Cologne V-6s (2.8s,2.9s,4.0s) since the late 70s...........In order to keep valve cover gaskets from leaking (they all will eventually, no matter what you do) I learned to ALWAYS circle the bolts with sealant AND apply sealant to the bolts themselves when installing them. There used to be really good neoprene Fel-pro gaskets, back in the 80s that seemed to last forever on 2.8s, but I don't see them anymore.
I will be doing this on our 4.0 Explorer soon, it doesn't look NEAR as easy as it was on 2.8s and 2.9s.