You're causing me to have flashbacks to all the oil cap issues I used to have on my B18. I agree with Ron about making sure the oil cap rubber gasket is fresh and the bayonnet tabs are tweaked as needed to maintain a tight seal. Those rubber gaskets often start out nice and plump giving a good seal, but as they age quickly dry out, shrink and get brittle. At the same time make sure you've got a good valve cover seal. The flared edge of a stamped valve cover can easily get a bit warped or misshapen if the nuts are unevenly tightened or overtightened. With appropriate blocks of wood you can hammer them back into better shape when needed. The original valve cover gaskets were a relatively thin cork that would also dry out with age. A newer, thick, good quality flexible valve cover gasket, possibly with some added RTV gasket sealent at the corners if you're desparate, will also help any oil seepage you see around the top of the block.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now