Well I hate to say this , but when we had a fleet of garbage trucks (including two Mercedes garbage trucks) we gave them a shot of either. A shot, always started,
20 below, 5am, always started.
Now I know that either is against the religion of many of you but, I also know you'll have sinned before and I'd rather give a shot myself than let a tow truck driver do it. Keep a can in your trunk incase you have the need to sin.
Now, if your out of town and the weatherman is saying 30 below and you know it isn't going to start in the morning . Get an extra key and let your diesel idle all night. One in the ignition and one in the pocket after locking the car. Just run it up the highway the next day and get rid of some of that carbon.