i have used those off the shelf products a few times, but they don't seem to work dramatically, however forte does the trick in my opinion. only available to trade but can be bought from ebay. with the same driving style and route, my mpg goes from 27.7mpg to 29.8mpg, 83000 miles engine
another tip, never ever use supermarket fuel, you defo get less mpg with supermarket fuel comparing to normal fuel from Shell, BP etc..