I've used 20W50 in the Chicago winter before changing over to 10W40. With 20W50 I found that it turned over slower in freezing temperatures. Once up to temperature it seemed to do well. With 10W40 I found that on short trips in freezing temperatures that a white sludge would build-up (under the oil cap). Once the car reached normal operating temperature the white sludge would "dissolve".
Since Mercedes recommended synthetic oil for 1998 and newer vehicles I've been using Mobil 1 15W50 for the summer and Mobil 1 0W40 for the winter. The advertisement on the Mobil 1 0W40 container states "Superior sludge and wear protection under severe, short trip operation". Time will tell on how it performs this winter.
Before Mercedes recommended synthetic oil for 1998 and newer vehicles, a local Mercedes dealer (Chicago area) said that they use 10W40 weight oil for the winter.
From the posts I've read Mobil 1 0W40 is the year round oil.
You might want to check out the Mobil 1 web site: www.mobil1.com
for the stats on their various weights.