While it might not matter to you kiwi's down there, in the Northern Latitudes it is important. Canada is rather North if my maps are correct, as is most of Europe, and that is even taught in the driving schools. Why you ask? Because of condisation. Gasoline is made up of water as well as oils stuff. (hence: Hydro-Carbons.) When a tank is kept near full there is less condisation and there full less water in the tank. In winter the water aclumulating in a tank kept at less than ⅓ full will acumulate enough water to freeze the fuel system. (As in car don't work.) Your engineer friends should have known this.
As for the additives, you haven't got a clue here as to the chemicle make up of modern petrolem products. Yes they do have additives, all of them do, and yours does too. They actual mixture will depend on the climate area where they will be consumed. Basic Fuel Management Courses cover this. -d