[njsteve]

On the warped intake idea, you can also have a leak on the underside of the intake that sucks oil from the lifter valley into the intake runners without ever giving any indication of a vacuum leak on the outside of the intake.

My vote is for the dried out rubber o-ring valve seals. After sitting a long time, those things can just fall apart on their own. They would also give an inconsistant oil down that would occur randomly and not all the time, depending on oil flow/engine rpm.