I fired up my 300sd yesterday evening after she had sat for two weeks through quite a cold spell. We had temps on the weekend down to -18 C.

When I started her it was -10 to -11 C or about 14 F. Started OK. You need to give a little extra gas pedal when they fire or it just seems to die .....my old VW diesel was just the same.

Much colder than that and I would use the block heater. When you want her to start, you want her to start!