I agree with the notion that it is more of an emotional decision to use the thicker oil in your engine.
Let's take a different vehicle altogether to use an example for a 'oil issue'. We will us my old vehicle - a 2000 Silverado truck w/ the 5.3L V-8. This motor - all 328 cubic inches of it is a clean sheet design from GM and forms the basis for the LS-1 design in the Vettes. Let it be noted that there is a class action lawsuit against GM for the 'piston slap' that is noticed by over 1/3 of the customers. The engine basicly 'Knocks a little' on start up. Let it also be known that in the owners manual for the '99 up Vortec truck line it is NOT recommended to EVER run 20w- 50 in those engines! They say stay with the 10w - 30 all the way up to death vally temps! I tried 50w mobil one in my truck just for kicks & it did not help!
My truck & also a friends truck started rattling upon start up at approx 10k miles. It is 'normal' to hear this on someones new body Tahoe, Silverado, Escalade sometimes. Let it be known that GM started spraying a polymer coating in the pistons in mid year '00 to 'quiet' them down for a while so customers wouldnt catch on to the sound. (for GM it just just buys 'em time for warranty work) The pistons are designed to expand when they heat up & that is why some think that thicker oil will provide the band aid/'cushion' for their engines. There has not been a rash of siezed up or damaged engines stemming from these power up knock noises. It is part of owning a chevy truck now & that is how it is!
check this site out - www.pistonslap.com
I'm not saying that GM engines are built tighter or MB's engines are built looser or BMW is better than MB or whatever. Like someone posted earlier, every car is different & there are to many variables to settle this opinionated debate on any fact?
A recent Car&Driver test of the new M3 says that they had to add 1 qt of oil at the 1,900 mile mark to show full. The press car is apparently clearenced looser for hp tests & burns more oil? Adding 20/50 wont fix that issue!
My MB owners manual for my C36 AMG says use 'use recommended oils' ........& thats it!
further checking in the manual says that "the dealer will have the proper info on what type of oil to use" Sounds like they want to get you at the stealership so a service writer can get you in to make some sales! My car came factory filled with Mobil One synthetic so that is what I have been putting in mine...10w/30 baby! I feel more comfortable knowing my main bearings are probably at least getting some oil at 5,900rpms & my oil pressure is not in the stratosphere.