+1 for the AOS.
Try removing the oil filler cap while the car is running. If it's being sucked on and hard to remove then the AOS is likely the culprit.
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-- 02 Boxster S
-- Black/Black
-- Sideskirts/PSE
did that repair about two weeks ago... suggestion is to have it done asap cause when it fails it doesnt tell you its coming... cheapest place in bay area is at micheal steade in livermore for $113.... couldn't wait for sunset to order the part and the other dealers on the peninsula are crooks...