So many people misunderstand the true meaning of "Octane Rating"........
It is simply a quantification of how difficult a fuel is to ignite. The higher the octane, the harder it is to ignite it. That is why low-octane fuel is recommended for such a great percentage of vehicles on the road; they have relatively low compression ratios and thus need lower octane fuel for it to be able to be consumed properly by the flame during the ignition process and subsequent power stroke. High compression ratio engines on the other hand, need higher octane fuel since the heat of compression would light the fuel prior to the spark occurring (pre-ignition) or the secondary compression that occurs after the spark lights the a/f mixture in the combustion chamber and the expanding burning fuel further compresses the portion not yet burning to the point that it spontaneously ignites itself elswhere in the combustion chamber (detonation). Both are deleterious to engine longevity. If you can remember when they used to advertise on TV for high octane gas there was tiny print at the bottom of the screen that said "for engines that benefit from higher octane fuels".
I can't remember the exact number, but diesel fuel is of significantly lower octane than gasoline, thus its ability to self-ignite due to the very high compression ratios common to diesel engines.
I am sure there are plenty of thread explaining this better than me elswhere in these forums, but they will affirm that high octane fuel in a 8.5:1 compression ratio engine is a complete waste of money.