Why would suppliers say we should use 000 oh I don’t know, maybe because they sell resin! If they said you can clean with 15,20,25, then they would loose out on sales of resin and RO filters. There is no regulation, there is no scientific proof it’s only down to trial an error.
As for increased MPG with high quality fuels, then you have been brainwashed by internet talk about it, along with supermarket fuel doesn’t have the additives. It’s all hearsay.
I have over 100k miles logged on a fuel app on all my vehicles, I’ve tried all sorts of fuel addictive, all makes and supposed qualities of fuel and it makes no difference, the only time you need high octane fuel is if your car has been designed or mapped to run on it.