I think modern fuels have made stuff like Redex superfluous, not that it was ever particularly fluous in the first place. I'd use it to clean oil based paint brushes, and to remove tar spots from paintwork.
Yes and no. Standard fuels do have some cleaner additive in them, which will do fine if your car does either mainly motorway or mixed driving and not much short distance work from cold.
For cars that have led lives predominantly doing short trips from cold, the standard fuels don't appear to be sufficient to stop gunk forming or to clean it up. The superfuels do, because they have much more (and better) cleaning additives, but the difference in price between the two is significant.
For an average 50L fill up (for an average 55-60L fuel tank in a C-sector car like a Focus), the difference between the price of standard UL at the supermarket and the super UL at Shell & Co (the supermarkets don't put as much cleaning additive in their SUL) is around 15p per litre.
That puts the cost difference of a fill up at £7.50. Apparently, for the cleaning effect to work, you need around four tankfulls on a reasonably gunked up engine/injectors.
The normal cost of an equivalent 4-dose (0.5L) bottle of Redex is around £8.50, but (pandemic aside) I've managed to buy it often on sale at around £5 (we'll say £6 now), so the cost for the same treatment is:
4x tankfulls of SUL: £30
4 does bottle of Redex: £6 - £8.50
Yes, the SUL does also improve mpg/performance, but normally only by about 5% tops for most cars. Over the same 4 tankfulls, that would take £1.50 off the cost, so still £20 more minimum.
Not sure if a bottle of Redex left for several years would be ok, as it's likely to separate somewhat into its constituent chemicals, even with the bottle sealed throughout. Not sure what (gently) shaking the bottle would do to help mix it all up again!
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