I suppose fuel quality could have an effect, but my theory especially on intake valve deposits is that oil vapors from the PCV system settle and build up over time.

If I had to guess, I'd say your Ford PCV system does a better job at filtering out oil vapors than say, a VW 2.0 TSI.

What I'd like to see, hypothetically, is a test where 2 identical vehicles under the same circumstances (same oil, fuel, etc), with one vehicle having a PCV system, and the other venting the crankcase vapors to fresh air. After 100k miles or so, you pull both intakes and compare. Not that I expect anyone to actually do a test like that, but it would be interesting.

James Kuhns
Owner/Technician
Auto Tech Solutions
Chatham, New York, USA