I am going to have to say that you have to use thicker oil. According to the temp/viscosity chart that I have a -30 weight oil is not approved by Mercedes to be used at any temperature.

__________________
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record