In Ronzi's sceario, I would personally value the Porsche maintained car over Mike the Mechanic"s receipts (or even Jim the Mechanic). Subjectively, I am not sure how much but I would buy the dealer maintained car, ALL THINGS BEING EQUAL.
I can tell you that among my Porsche customers, that is a tendency of say, maybe half my buyers (very unscientific I know).
So, I am clear that there is no right or wrong answer to this question. It is, like many issues here, one of how one chooses to spend their time and money.
Hey, I was watching the ALCS when I could have been changing the oil in my Box!!!!!!!!!!!!!!!