Use posh petrol in any car with a knock sensor and you'll get better miles per £ than ordinary unleaded. I'm paying about 4p or 5p more per litre but getting about 10% better mpg so well into profit.
You also get all the cleaning additives that Wynns etc charge so much for. If the car does not have a knock sensor you wont see an mpg improvement other than from the cleaner injectors etc.
If you go to France their basic petrol is very low octane and diesel is low cetane so bottled boosters are well worth using to avoid stupidly low mpg