I have never been as cynical about premium fuels as many on here seem to be and have been increasing my use of them over the last decade. I have seen a number of tangible benefits from using them as you describe. Fuel now is not as good as it used to be with all the methanol and vegetable oil added and I think premium fuels combat the ill effects of this better than regular.

Currently running on Costco premium diesel that was 12p /L less than my usual V Power, I got 62L into my 70L tank so the saving was a fair bit. Too early to say if its any different.

As my recently-acquired Pug 306 hasn't done much for a few years, I shall bung in 10 litres of super on Saturday to see what happens ....

T'other day, was away from home and fuel light came on, (Nothing unusual there then) but was near a certain supermarket selling fuel, and offer was spend £60 and get 10p off a litre, so decided to fill up with Momentum on the spur of the moment.

One of the characteristics of the MG?GM 1.5 engine is that it feels faintly (and i mean faintly!) when cold and very very faintly "lumpy" with an occasional intermittent hesitation when warm

After a week on momentum it feels as if it is running sweeter, so will continue the experiment, as now we are with Shell Energy at home, we get a 3p discount on fuel..

I have read the pros and cons before, and have usually used Asda, as it is a very high turnover petrol station very close to us.

But will be interesting to try for a while.

From reading many comments over the years on this forum on this subject, I suspect its a YMMV time - some cars will love superfuels, others (like mine) seem to not really notice much, however I suspect on the defining factors is HOW the car is used and maintained - lots of short urban trips from cold and/or a lack of decent maintenance (not implying anything here ORB), then the engine is more likely to gum up with deposits of carbon via fuel that isn't completely burnt from running a rich mixture more of the time.

The branded superfuels with their extra cleaning additives etc would likely help cars in those circumstances far more than just standard 97-99RON from a supermarket filling station, but obviously you pay a large premium for doing so. This is why I prefer to occasionally use a (seemingly) proven fuel injector cleaner additive (especially when it's on offer in the shops - far cheaper than using branded superfuels), but then my car will get the full benefit, especially for the cleaning of the valves, as my car is an older port-injected engine, rather than a direct injection one.

I noticed about a 3-5% (using the brim-to-brim method and comparing very similar usage at the same time over a few years) increase in mpg (i.e. 1-2 mpg) using each method, but much else, probably because my car does the vast majority of its work on fast flowing roads for decent length trips, meaning its mostly warmed up running optimally and thus the injectors, valves etc are likely to be reasonably clean.

Very little difference as a result from supermarket to branded fuels. I use the injector cleaner maybe twice a year at most, especially coming up to service and me going (driving long distance) on holiday.