I went to Vermont last week at 7F below and (with a block heater, though) my 300SDL started and ran just fine. A fuel heater is required only if you want to run your car on vegetable oil.

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'99 S420, 155k
'91 VW Vanagon GL, 150k
'85 VW Vanagon GL, 120k
'82 VW Westy, aircooled, 165k