Your best bet is to go to Mobil1.com and see which weight would cover that temp. range. Normally 15W-40 is the universal conventional oil weight. Your owner's manual will list various weights based on temps., the problem is those are based on conventional oil. I checked the owner's manual for my '83 TD (same engine) and the chart only goes down to -4 f. and for that it list 5W-30. Again, this is based on dino oil. I also own a 1987 TD and I recently switched to Mobil 1 15W-50, but I am down here in NJ, where it generally only gets down to 20 degs. in the winter.
BTW I mention Mobil 1 because it is more readily available. You may be able to use 0W-40 which you can sometimes find at a Walmart.

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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