Years ago this was done by the driver at the gas station. Today, at least in Germany the diesel fuel is already premixed. It stays liquid below -20 Celsius.

The problem is that Diesel is getting semi solid on low temeparature. Petrol avoids this effect.

In the U.S. there are two common distillation grades of diesel fuel: D1 and D2. D2 is the more common #2 diesel fuel sold at most U.S. pumps for most of the year. D1 is a lighter distillate similar to kerosene and jet aviation fuel. In fact, there is significant overlap in the HC components of D1, kerosene and jet aviation fuel. D1 is often sold as winter diesel fuel in cold climates in the US. It has a higher cloud point and a higher melting point and is often sold as "winter" fuel. It sometimes (but not always) has a higher cetane number. It typically has reduced lubricity, lower kinematic viscosity, somewhat lower energy content, and sometimes is more expensive. Commercial diesel equipment operators will avoid using D1 for these reasons except at very low temperatures or they will blend D1 and D2. Some commercial operators will use fuel tank heaters that (recirculating engine coolant) and/or heated filters to allow operation on D2 in colder weather and avoid D1 or D1/D2 blends.

I have heard of mixing petrol with D2 as a quick fix to lower cloud point. Using other light distillates with better cetane or using cloud-point suppressing fuel additives would be a much better idea. Petrol/gasoline has very low cetane number, thus blending with diesel would lower cetane, increase ignition delay and thus could cause a considerable increase in white smoke during start-up in very cold climates.

Thanks a lot for answers.