Here's my 2 cents based on my personal experience and the other cars in my family that I'm responsibility for (inlaws, etc).
I run the thinnest weight that I can while still maintaining reasonable oil pressure at idle. My reasoning is that I want plenty of fresh oil flowing through the bearings and carrying heat and crud away quickly. I also want to avoid unnecessary power loss from pumping thick oil.
I have never cared what brand it is. I buy what is convenient. I would run 10-30 if you get .3 bar of oil pressure or more at idle (while hot) per the MBZ manual. If you're pressure is below that I'd go with 10-40.
Experience is something you don't get until just after you need it.