I've used 15W-50 Mobil 1 in my '91 300E in both Minnesota winters and Arizona summers. Remember that this oil has the same pour point as 10W-30 petroleum based oils. This oil was what Porsche told their owners to use before 0W-40 Mobil 1 was available in the U.S.
I think that you could use this oil all year round in DC. MB says you can use it down to 4F.
Also, MB has told their dealers not to use 5W-30 or 10W-30 Mobil 1. There's some doubt that these oils are true synthetics now.
I've been able to find (occasionally) 15W-50 Mobil 1 packaged in 5 qt. containers at Walmarts at a price of around $3.60/quart.
One more thing, Mobil puts more anti-wear addititives in 15W-50 than in their other weights.