0 octane
http://en.wikipedia.org/wiki/Heptane
100 octane
http://en.wikipedia.org/wiki/2,2,4-Trimethylpentane
Notice the only difference is the shape of the molecule.
Typical gasoline contains a slew of different shapes that average out to the octane you hope you put in the tank.
The accessibility of the gasoline molecule to oxygen radicals is affected by shape, as well as the packing ability of the gasoline molecules, which influences vaporization, and kinetics when reactions take place.
At the end of the day its all fat without heavy carboxylic ends. But just like in the body where some fats are healthy and some are not so much...