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. That is because the engine invented by a German called Diesel, requires no spark plugs. The diesel engine will ignite and combust when the pressure increases so that the heated "plug" makes it explode. Here we get the name "Diesel" - since the same principles are used in diesel engines today. 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.