I dont know if anyone else has had problems with heating oil in Spain but I have. I have had oil heating and water for some years now and experienced build up of water in the steel tank and also coking up of the boiler jet.
An engineer recomended an additive for the fuel a couple of years ago and I have not had any problems since. In fact I think I am using less fuel than before and with no smoke.
The additive is Fuel Set and I buy it online from liquidengineeringspain.com Hope this can help others.