D2 is a refinery abbreviation for Gasoil. It is the second distillate from the crude, and can be used without reformers and additives. So, the first engines used D2 as fuel - before petrol cars as we know them today was invented.However, automotive diesel that you fill has additives that the refinery will add to make the engine more efficient and also easier to start in the winter. If you read the small print, you will see that diesel changes "flash point" in the winter. It also has additives to absorb water that condense as you have it in your car (same as petrol) - but since the diesel is injected right into the cylinder, the ice will destroy the nozzles long before the engine. If you use summer diesel in the winter, you will get better mileage but your fuel pipes may freeze and can also burst, and the wax makes the diesel flow thicker.