Coldest I've ever fired mine off was -7F after 20 seconds of glowing with 15w40 dino rotella, took about 12 engine rotations to begin firing, then about 3-4 more to get it going, but it did it!
This was last year. I am running Mobil 1 5w40 synthetic now, and it fired off last week at 16 degrees with a 7 second glow in less than 3 cranks.
Synthetic makes a huuuge difference. I haven't used my block heater yet, I do when its below 15F out, I just plug it in overnight. It takes forever to get heat though when I don't use the heater, so I may just start doing it in general....I have a hose heater, if I run it all night I have heat out of the vents within a minute of starting it.
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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)