I looked in the Mercedes oil guide and it says that 20w50 is good to +23 degrees Farenheit. The next rating, 15w is good to +5 degrees farenheit.
I would recommend atleast 15w just to be safe. Mobil 1 synthetic has 15w 50, which is what I use, and would seem to work well for you also.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)