Back in 1982 I bought a VW diesel Rabbit. This was back when only a few city stations sold diesel and the winter 'blend' was not real reliable. I lived in Iowa and I could count on fuel jelling problems when the temps went below zero. Trust me when I say you don't want to be rolling down a highway and have your car sputter to a stop in below-zero temperatures. There is absolutely nothing you can do except have the car towed. But who had a cell phone in 1983 ??? So it could be a dangerous situation.
Part of the problem was that these cars would easily get 40-45 mpg around town (I documented 60 mpg on the highway) which meant a 10-gallon tank lasted forever. So you could fill up in December and still be running that fuel during January when the temps would be significantly colder and the fuel needed a different blend.
After having the fuel jell up a few times I added an in-line fuel heater (thank you JC Whitney) just upstream of the fuel filter and started adding a shot of anti-jell every time I filled up. But during especially bad cold snaps I added several gallons of gasoline to the fuel tank to thin out the diesel. As I recall, the owner's manual talked to this subject and even stated the maximum amount of gas that could be added. Definitely got strange looks at the station when I put fuel from 2 different pumps into the tank.
Apparently, the VW HPFP was not affected nearly as much as the ones we have in our trucks now. I know that driving with even a little gas in the tank will cause big and expensive problems in my current truck.