Well said frinesi2. The only thing I would add is that gasoline contains butane also, and the amount differs depending on summer/winter fuel mix. There is a higher proportion of butane in gasoline in winter to increase vapor pressure and, as a side note, is one reason prices tend to be lower that time of year.
I agree with your assessment of octane, timing and their effects on low rpm torque and fuel mileage. This is why I run 87 octane on long highway trips, but switch to a higher octane when the tank will be used for more city/stop-and-go driving--especially in the summer.
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