I use 15w50 Mobil 1 in CO temps in the single digits are common. I have had no problem in fact it flows well even at this temp.

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Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)