I just changed the seals on one cylinder of my rebuilt 318 with less than 5K miles on it. (it was a purchased rebuilt).

I had the same issue as you, one cylinder oil fouled. When I was changing the seals I found that the top of the intake valve had a sharp edge and I believe that it cut the seal when it was install.

I cleaned up the top edge and installed a new seal with the tube like tool that came with them.

No more oil fouling.

:-)