I'd usually fill up at Shell, apart from a very good looking and lovely assistant called Eventa (worth popping in for anyone living in or around Skelmersdale), I also notice how I do seem to get more per tank and the car does run smoother. Unfortunately since April my local Shell has been having a refurb, so Asda is my only other choice, so its a additive in with it, the 2 stroke idea I remember seeing a while ago, can't actually remember the specifics of what it does, but I'm guessing a quick search on here will reveal all.. Done a bit more research and 15,000 seems to be the common consensus, I do around 14,000 a year, so 12months or so between will see me right.