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.
The Medic
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Wednesday, December 6th, 2017 AT 7:26 PM