I asked this question of Honest John recently, when I bought a petrol direct-injection car. His view was that yes, you get a cleaner burn from the likes of Shell VPower and BP Ultimate, so that will help to keep valve seatings and stems clean. I'm fairly sure (though ready to be corrected) that Shell uses some gas-to-liquid (GTL) in VPower, which I'd certainly expect to be clean burning. Don't know about BP.
Given the relatively small price premium for these petrols (when compared with other motoring costs), good experience with VPower diesel in my previous car, and my intention to keep this new one for a long time, I am quite happy to carry on using VPower/Ultimate.
I've been 'testing' the use of superunleaded (V-Power) for 4 fill-ups on my (nearly) 12 year-old (though only 63k miles) Mazda3 1.6 petrol (N/A), and noticed a slightly better (smoother) pick up as time has passed, but only a 3-5% increase in mpg (being used in the same way as before in the same conditions as previous years), despite the fuel costing about 8-10% more (only 5% more if I bought it at supermarket filling stations, but that doesn't have the 'cleaning' additives). I might see better performance in the same rate and no increase in mpg if I gave it more right foot, but I'm not sure, as I haven't been able to find out whether my car has a knock sensor.
I've also tried out Redex, and given it appears to achieved a similar result at a far lower cost (injector cleaning mainly - I'm fortunate that I don't drive short journeys that will significantly increase carbon deposits on the valves etc): essentially £5 (I got it on offer, normally £8.50) for a four-shot (one per full fill up of fuel) bottle of the stuff as opposed to £20 for the extra fuel over the four fill-ups in total.
I think if anyone does lots of short trips where the car doesn't use the full range of revs and gears and/or isn't fully warmed up (e.g. trips to the local school/shops of a mile or two/5 mins), then I would use superunleaded with the additives once in a while (say 3-4 fill-ups before the service/MOT or long [important] trips such as holidays), but not all the time, and to be honest most reviews of Redex seem to be positive, though mainly at reducing clogged fuel injector nozzles only.
Both help more on my car as its a non-GDI engine, and I suspect, like with diesels with DPFs, taking a GDI petrol-engined car for a long drive at motorway speeds, coupled with an occasional 'Italian tune-up' (only once fully wamred up) is probably just as beneficial. For my car, I'll probably use the bottle of Redex once a year coupled with an occasional 'Italian tune-up'.
I'm not trying to advertise either BTW - just giving an opinion based on my personal experiences - I'm sure all the similar products give relatively similar results (read reviews on the sites that sell them and here for the fuels).
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