Petrol is rubbish in this country anyway, It doesn't matter what grade you use.
Petrol in the UK is considerably better in quality to that found in Spain for example, I know - my sister runs a Spanish registered car which she's learned the hard way to always fill with Shell,rather than at "El Gringo's" garage down the road from her apartment!
Back on topic....
One fact is beyond dispute – Supermarkets and the main oil companies get their fuel from the same refineries.
Now if the ‘magic’ ingredients that, say, Esso for example, added gave better MPG than Supermarket petrol do you not feel they would get it verified by an independent body(AA/RAC motoring magazines, [TW]Fox

etc) and you would never stop hearing about it in the media?
An objective test would very easy – get some identical cars put in different makes of fuel and drive them in convoy swapping drivers every so often.
Also if the ‘magic’ ingredient had any proved advantages do you not feel that the rival suppliers would have the fuel analyzed and that ingredient added to their fuel?
There is absolutely no proof that brand X gives better MPG than brand Y and the oil companies are very careful not to claim so – lest they contravene advertising regulations. If anybody could prove me wrong, I'm happy to be preached to!
