Wow, an oil question, this is a first.
Just joking, simply stick with what your owner's manual says for the temps that you are living in. My '83 TD owner's manual says that 15W-40 is the recommended weight (viscosity) for temps down to 23 F. So since you are in Charleston, that would mean 15W-40 would cover your local year round temps., for sure. Here in CT, for the Winter I switch down to 10W-40 then go back to 15W-40. for the Spring/Summer I use the same oil in both the diesels and the SL, which happens to call for the same viscosity.

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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