heating oil is basically Gas Oil or red diesel, same stuff that plant vehicles and ships and things run on. It has the red dye so that Customs and the police can tell if you have been using it instead of DERV (diesel Enginged Road Vehicle fuel, or white diesel) in a road vehicle (Much more tax on DERV).
its not the only thing that will run your boiler (Vegtable oil will work too) but it is probably the cheapest.