First mistake was asking the service advisor. My experience is that 1/2 are good and know what they're doing and the other 1/2 can't find their own butts with both hands. (And at my dealer these are invariably the service advisors who have the atrociously large butts!)

The gasket is just a couple of bucks.

My parts person doesn't ever give a $hit WHY I want to buy the part!!! I might want to use the gasket as a coaster on my desk or an earring for all he cares.

My V70 turbo did this, the gasket was hard as a rock and for a couple of bucks I solved the problem. If you replace the gasket and it solves the problem then it isn't likely the crankcase ventilation system and you've saved the $100 diagnostic fee, not to mention the cleaning (of your wallet? -don't forget the function of the service advisors is to SELL Volvo service!). If the gasket and/or the new cap don't solve the problem, then you can pursue it further. Of COURSE it could be the crankcase ventilation system or blow-by but I'd pursue the cheap simple stuff before the expensive complex stuff!

-Punxsutawney Phil