That is utter nonsense, they all contain additives these days, you are paying for higher octane..
Depends what you mean by the word 'additive'. Bit like saying all cakes contain 'ingredients' so why pay more for a Fortnums cake than a pikey Spar one. I'm with Dem on this, providing you use the right expensive fuel. Shell V-Max and BP Ultimate for me thanks where possible.
All 95RON fuel comes from the local depot. Doesn't matter what brand the station is, the fuel is from the local depot, so BP sell Shell 95 and Esso sell BP etc. It's only when you get to the 'Super' grades of fuel that the companies refine and sell their own, and Shell's is the best in terms of performance and quaility, much like Mobil One can't be beaten as fully synth. Their chemical additives to boost the octane number and detergents are superior as they have done more rersearch for longer, simple as that that. So you pay for that, for the increased transport costs of moving your own fuel from your own refineries to your own staions and for the R&D that has gone in. Tesco 'super' uses ethanol to boost the research octane number (RON) closer to 100, it doesn't have a great thermal capacity but it is cheap. Using a more complex and pricey substitute will allow you engine ckleaning and lubricating benefits, but then you pay more.
If you are lucky to have a car with a knock sensor (which advances or retards ignition), then the higher octane reading wil boost performance (as your fuel can be compressed more before it ignites generating more power). Even without, the fuel is better quailty and anecdotally people generally report better economy and sometimes performance. All the cars get 98 except the poor old VW.
All octane is, is an 8 carbon chain hydrocarbon (1 methane (household gas), 2 ethane, 3 propane (LPG), 4 butane (lighters), 5 pentane, 6 hexane, 7 heptane, 8 octane) and what we use for our engines happens to be predominantly octane just due to it's boiling point and reactivity. The Reserch Octane Number (RON) describes, in terms of a percentage, what the ratio of octane to heptane would be to get that specific level of retardation. So 98RON is the equivalent of 98% octane and 2% heptane. RON isn't the energy content of the fuel, it's the fuel's tendency to burn in a controlled way rather than exploding in an uncontrolled way. Where the octane number is raised by blending in ethanol a-la Tesco and other cheap 'super' fuel's, energy content per volume is reduced. So you get your increased resistance to detonation (more power per stroke) but a lower energy output than a decent fuel would have. A net win then, but still not as good as Shell or BP fuel.
If anyone cares. Slow day and been to the pub for lunch.
If you can't be bothered to read it, it basically says 'you pay for what you get'