Maybe this will help you visualize how it all works. If when you removed your valve covers and some areas were clean (the part of the cover over the rockers) and other areas were grimy with dirt and grit etc. (the part of the cover NOT over the rockers)then that would be the tell tale that perhaps your rocker arm shafts are leaking into the exposed part of the engine under the valve cover.
http://forums.pelicanparts.com/porsche-911-technical-forum/684592-rocker-shaft-seals-cleaning-up-mess.html