We have somthing called fuel duty, we also have VAT at 20%, when buying oil as a lubricant you pay VAT which is 20%of its total price, because diesels run on prettymuch any combustable liquid our government dictates that if we re use the oil as fuel then you need to pay the fuel duty which is 60% of the cost of 1 liter of pump fuel and because the oil your running is now a road fuel it needs the duty paying