There seems to be a perception (lets blame the oil companies marketing) that a higher octane (and therefore more expensive) fuel is a) better for your engine and b) will lead to more power or fuel efficiency.
In many cases, the above are simply not true. 98 RON gas, for example, has no more 'energy' in it than 91 RON. The higher the octane rating, the less likely the engine is to 'knock'. The higher the RON, the higher the 'auto ignition' temperature of the gas. Same energy, same burn time, different ignition points.
The reason some cars require a higher octane gas is that they are tuned for it, often because the engine has a higher compression ratio and therefore a lower octane fuel is more likely to ignite early and knock.
Many modern engine management systems will compensate if you put in a lower RON fuel but modifying engine timing at a cost of some power. So in actual fact, if you put 91 RON fuel in a car tuned for higher, you will get a reduced power output (in a modern car). The marketing around 'more power, smoother acceleration' etc. is all based around the fact that this will happen if you are using a lower octane fuel than your car is designed for.
Unless you drive a high performance vehicle, or a euro designed for it - Your car is likely no better off with a higher octane fuel - nor is your wallet.
My bike dealer told me to put in the 'highest octane fuel possible' and that the bike will 'cough and splutter' if I put in dirty 91. In fact the bike manual suggests 91, which means that the engine won't be designed for 95+, and therefore I would just be wasting my money.
91 RON fuel is no 'dirtier' than 95. Only difference being some oil companies will add 'cleaning' additives to some of their 'premium' fuels, but their effectiveness has not been proven. There has also been suggestions that running a higher octane fuel than required will lead to deposit build ups within the engine, but that is beyond my understanding and possibly incorrect.
I didn't know about how the Octane rating worked until a few months ago when I read about it, and before then I too believed the marketing hype around premium, higher octane fuels.
I'm keen to know if anyone has been running a different kind of fuel than what the manufacturer says to in your vehicle, and what you have noticed.
Don't waste your money on fuel you don't need people!