Okay, LOTS of misinformation in this thread!

Here's the deal - ALL gas burns equally cleanly - just at different compression ratios. Octane ratings are used to determine at what compression the gas burns completely.

(I might have this next part backwards, I'm sure someone will correct me)

Higher octane fuels will burn completely at higher compression ratios, while lower octane fuels will burn completely at lower compression ratios. Again, I might have that backwards, but that's generally how it works.

If your gas doesn't burn completely, it'll collect in your catalytic converter. If it burns too quickly, it'll make your engine knock.

Basically check your owner's manual to see what the compression ratio of your engine's cylinders are (and which gas they recommend), and don't listen to BS from fairweather mechanics - or (ironically) BS on Internet message boards.