Have you used an inspection mirror (or your hand) to see if the valve cover was leaking down the back side?
This may be an unnecessary review for you.
I uploaded a 258 diagram of its rear main (below).
When I have these apart I use brake cleaner to get all the oil off of the rear main cap and the opposing part of the block.
I lube my upper half and install it.
I brake clean the mating tips (so they are oil free) that showing and let them dry.
This will let the bottom half's silicone adhere better to the top half.
I install my bottom half of the seal and lube the lip only, being careful not to get oil where my silicone must seal. I silicone the chamfered edges of the rear cap's sides. This keeps oil from weeping out of the little hole/ crack that forms in the corners when the cap is seated in the block.
I "painted" the silicone portions in red in my diagram.
Other than that,
A crack in the block or a leaking oil galley plug hidden under the bell housing?
Other 2CarPros experts may jump in with their ideas too.
Keep us posted.
Images (Click to enlarge)
Wednesday, December 6th, 2017 AT 7:26 PM