Mine came with a block heater, but where I live it's inconvenient to use one. I've started mine at 25F without it, but it takes "work". As others have said, it depends on how well your valves are adjusted and how the compression is. Also, the weight of motor oil (I'm going to put in 10W40 instead of 15W40 for the winter) and whether you've got any kerosene in the fuel tank (my owners manual recommends an 80-20 mix of diesel and kerosene for 25F, which I didn't do last winter).

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1982 240D automatic
Odometer quit at 307K...about 325K now