You should be fine with synthetic oil for those temps. Look at your owner's manual for what oil to use in those anticipated temps. Per the MB Factory approved service products list, for temps that low, the following are recommended weights; 0W-30, 0W-40, 5W-30, 5W-40 and 5W-50. If your car doesn't already have a block heater I owld definetly get one installed. The block heater actually sits in one of the block's freeze plugs. When plugged in it heats the water. As water heats up it begins to circulate, thereby warming the whole block.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member