Here's my $.02 worth...at least on watercooled engines. I've used
Mobil1 exclusively on my 86 GTI. It now has 205k miles on it. One
day, when the engine had 150k, and I had nothing else better to
do and had an enquiring mind (and didn't want to read about 
Michael Jackson poking young boys in the Enquirer), I pulled
the head off of the GTI.

The cam area was nice and clean, there were no discernable wear
marks on the cam. No I did not measure it. I did a quick check
on the cylinder walls by running my fingernail up and down
the bore. Basically smooth with only the slightest ridge at
the top of the bore.

Oil pressure has dropped only about one or 2 psi through out its
life, and I currently go through a quart about every 2,700 miles
as opposed to the 3,000 when I first bought it.

As soon as the EuroVan went for its first 1,500 mile (my choice
not VWs) check up...I had the dealer drain the oil and put in
Mobil1. I dump the Mobil1 about every 4,500 miles or so...just
about the time it needs to have a new quart thrown in!  8-)

Ric