Positive crankcase pressure is easy to check with the engine idling. Just cut a finger off a latex glove and insert it over the dipstick tube. If it gives you the finger, you got problems. If it trys to get sucked into the oil pan, grab it and say thank you.

You might have a leaky turbo oil return line seal. About $20 plus labor. That will leak onto the hot exhaust pipe when the turbo is engaged.

Check the oil cap seal, replace it if it is hard as a rock.

Check the alignment of the rubber seals between the hood and the lower wiper motor cover. This is supposed to keep engine bay vapors out of the cabin. BTW, check/replace the cabin filter if it hasn't been done lately. FCP has a charcoal version.
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My name is Klaus and I am a V♂lv♂holic

Our 05 S80 2.5t was doing this. It turned out to be that the vent hose that runs from the breather box up to the top of the engine was torn open just above the breather box. It's kind of hidden under the intake manifold, but I think you can see it if you peek down between manifold runners - like between runners 1 and 2 or between 2 and 3.