More on this topic - I ordered a replacement rubber seal for my oil cap - I bought the Porsche brand, not aftermarket. I found that the original part #911.207.901.01 is no longer available, but Pelican showed superseded part #930.107.272.01. The seal appears to be the same size as the old rubber seal I removed (which I am now thinking may not have been original), however the cap doesn't appear to fit very snug even with this new seal. In fact, I put both seals on the cap and it fit more snug (surprising I was able to easily put the cap back on with both seals in place). I guess I am now wondering if the correct rubber seal is no longer available and the superseded part is not to same spec? Looking at photos in this thread, it looks like both the cork and rubber seals extend from the outer edge of the cap all of the way to the beveled edge of that center 'bump' on the cap, whereas my seal shows a gap (see photo above).