You are off on this one Dutchman. Gasoline molecules have between three and twelve carbons(mostly seven through eleven). 92 octane gasoline is a blend of the various size molecules that act(ignite) as if it were a blend consisting of 92% octane and 18% heptane. 87 = 87% octane and 13% heptane.

Problem in today's fuels is that the octane level is increased by using various additives. Some use alcohol to do this, others use MTBE.

What makes you say higher octane fuel runs cooler? There is no appreciable difference in the heat of combustion from 87 octane to 92 octane gasoline. If anything, I would expect higher octane gasoline to burn hotter. Funny cars run alcohol right? They run extremely hot. The temperature of your engine has a lot more to do with your ability to cool it than the fuel you use.