I remember hearing something about the Germans burning fires under their tanks to get them started during cold World War II winters. However, I'm not sure if those were diesel or gas motors...

Along the same lines, in a pinch, you could do as kerry recommended, and use charcoal underneath. You can also keep a small camp stove in your rig. Those things output some serious BTUs, and would heat up your oil pan in no time. It's not a bad addition to an emergency winter survival kit.

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-1985 300TD - 324k mi on chassis. MB "Tauschaggregat" 617 motor + 4-speed conversion
Alaska Roadtrip 2009
-2002 E320 Wagon - 197k mi
-1998 E320 Wagon - 310k mi - retired