I don't have any gauge data to back this up but I can tell you this,
I've been running 15W50 Mobil 1 in my Porsche 944. Just last week I
started a new job which involved a truly grueling commute around the DC
beltway. After sitting in stop and go traffic for a while in 90 degree
heat I was getting serious detonation while trying to accelerate. I
blamed it on Shell gas (I usually run Chevron with no problems but
anymore I don't regularly drive by a Chevron station so I have to take
what I can get.)

I changed my oil to Mobil 1 0W40 over the weekend. Not only does my car
feel much better, but oil pressure has dropped only marginally, and
guess what - despite being stuck in miserable traffic trying to get home
from work today, ZERO detonation. Now it was only 80 degrees not 90
today, but still... I have to wonder if the oil didn't have something to
do with it. I didn't fill up my gas tank until I was almost home, so the
fuel in the tank was the same as that I was running the latter part of
last week.  Maybe the lighter oil weight allows the oil to move through
the engine faster and/or more volume is being squirted onto the bottom
of the pistons?

I suspect when I first got the car, it had 10W30 in the crankcase as the
oil pressure readings were low enough that I got a little nervous when
the car was fully warm.  With the 0W40 it seems close to "just right."
Now the question is, why are Xw40 synthetics so difficult to find?

nate