I dont buy into these additive products at all since using castrol lead replacement stuff which did jack ****. When MOT time came round recently, the place I took it to offered me a bottle of 'Dipetane' to try (any pay for!!!)
I told the bloke my feelings and he was a bit suprised, so much so he stuck a dose in the tank for nowt and told me to go fill her up. I have been back to buy a bottle!! Really does seem to do what it says it should. Apparently been available for years, but not been easy to get ahold of for a while.
Not badly priced at £12 for 5 brimfull tank worth!