Seen quite a few topics on here already asking this question. Personally, i try and use Super Unleaded all the time, and, if im near a Shell garage i'll always use Optimax as i definately notice a difference with my MPG. Unfortunately, there isnt a Shell garage in North Devon, so i can only fill up with it when im elsewhere.
I noticed a 3mpg difference driving the same journey from normal unleaded to Super. Some people say it is a "Placebo" , that its just a marketing scam and doesnt make a difference, but with my GTI it runs so much more smoothly on Optimax