[grannyknot]

Can't believe no one has touched on this; 91-94 octane gas may get run through a finer filter, may have some extra detergents and all the rest of the sales hype.
The difference between 87 octane and 91 octane is that 91 is harder to ignite so in a high compression engine the gas won't preignite before the optimum time in the compression cycle.
That's it.
Xylene and toluene are added to make the gas less volatile, harder to ignite.
The computers in cars these days can compensate for the wrong gas being used, so if the station you are at has run out of high test you can throw in a few bucks of regular to get you to the next gas station without hurting anything.
Does 91-94 octane gas have more power, more BTUs? Nope.
Is the price of 91-94 justified? Absolutely not, the extra additives are not that expensive.

If your engine is high compression then using 91 is a good idea because that is what it is designed for.
In my opinion, it is not better, it doesn't provide more power, most of the sales hype around it is just that, sales hype, the profit margin on 91-94 must be enormous.