Have you ever noticed that when re-fueling at a supermarket they sell "city diesel" while most fuel stations sell normal diesel and Shell have their optimax type along with BP who do one too. I read somewhere that there`s very little difference between any of them. I think my Saab runs smoother on optimax but the fuel consumption is no different. Does city diesel harm your engine if you drive up and down the motorway or go on long fast journeys? Does anyone out there put in an additive that makes a difference to their cars performance? I have a Polish friend who was mixing veg oil and diesel 50/50. It was ok until the cold weather arrived!