I use the Mobil1 0W-40 exclusively, and I will venture to say that just about everyone else is probably doing the same. This oil weight it not a common weight, but it is readily available at the auto parts stores.
Trying to outguess the factory specified oil is not something I would recommend. Sure, you can analyze the weights available and temperature range your engine will experience and maybe come up with a weight that seems better suited, but it just ends up being a guess since we weren't there during the engine development. You should just stick with the factory specified.
I believe that the 0w-40 is a thinner oil than most available and it has probably been specified because of the tolerances of the MB engine…in fact using a thicker oil (which is probably most of them) would not be a good thing!
I change my oil using a vacuum pump canister system called the Topsider. With this, changing the oil in the Crossfire is a real joy.
Here is a website explaining how to do it...I got the idea from somewhere on this site:
http://www.slk32.com/pages/oilchange/oilchange.html