The valve covers stay dry, but a strip of duct tape run along the head/block crack gets oily on top of the tape? Maybe the intake manifold end gaskets are leaking, and the
oil is running along the crack.
Clean it all up really good, then start looking for the source again. Don't drive it, just idle it in the driveway or rev it up if necessary, but don't drive it, that will blow the
oil all over the place and you won't be able to tell where it is coming from.