from my understanding, the octane number accounts for the percent of hydrocarbons in the fuel.. if you're using 91 octane, it means it has 91% iso-octane (a type of hydrocarbon).. the higher the octane, the higher compression the fuel can handle and the more resistant it is to knocking.. and the lower the octane, the higher chances of having your motor messing up on you and that's a way you can experience rod-knock..
Chevron w/techron 91 octane all day for me! worth it no doubt..
cleanses and keeps motor running right