To complete the On Topic Subject of the Cause, Origin ...and Cure for this Problem:
Back when I was designing and building “The Franken-Oiler Machine” as a Functional Electro-Hydraulic Engine Pre-Oiler…
http://s557.photobucket.com/user/60dgrzbelow0/library/ENGINEPREOILER?sort=3&page=1
… I also modified a GM 4.2L LL8 Valve Cover to sport some Glass “Viewing Windows” on top of the Oil Vapor Collection Chamber in the cover so I could observe the performance of the Valve Train in real time as the Fresh Motor Oil was being distributed under pressure by the Pre-Oiler Machine.
At that same time… while removing the Old “O” Ring Style Valve Cover Gaskets and replacing them with a New Set... I could NOT believe just how much they had all flattened out over time… so much so in fact that when viewed in profile ...you could see why GM only required a mere
87 Inch Pounds of pressure to slightly squash down the Gaskets… as they literally
completely suspend the Valve Cover up and off of making direct contact with the flat flange surface of the Aluminum Head floating on those “O” Ring Style Gaskets.
Take Notice that even the “Trapped Fasteners” themselves...are completely isolated from ever making Direct Contact with the Valve Cover Hold Down Flange via Black Rubber Grommets.
But with time, temperature and the wear & tear caused by Engine Heat and Vibrations… they eventually 'Give Up The Ghost' and after losing their “Plump”… they fail to separate the actual Composite Nylon Cover-Lid from making contact with the Aluminum Head. By then… Hot Motor Oil has long since migrated from the inner Valve Cover Gaskets down inside of the Spark Plug Wells...where it can cause all sorts of mischief.
Likewise ...this very same phenomena occurs with the Plastic Intake Manifold… that oddly bounces and jostles up and down due to the “Teeter-Totter” of the weights of the Throttle Body on one side… and the PCM bolted up on the opposite side. In time… ALL of those Gaskets Flatten Out and Fail...and this results in the Fasteners loosening, too as the Gaskets give way. Here are some very illustrative images of what these Gaskets look like in profile… when Brand New: