If the engine has adaptive ignition (retards spark in response to pinging/detonation) then using lower octane fuel in a higher-compression engine may not result

If the engine has adaptive ignition (retards spark in response to pinging/detonation) then using lower octane fuel in a higher-compression engine may not result in engine damage - just less power. OTOH, using higher octane than the manual / engine specs call for is just a waste of money for no extra power - but is not damaging to the engine. One of the less-noticed issues with ethanol as a fuel is that it contains/provides ~33% less energy than gasoline[1] - so saving n% per gallon at the pump may cost you n% in mileage down the road. [1] http://www.eia.gov/tools/faqs/faq.cfm?id=27&t=4 http://www.eia.gov/tools/faqs/faq.cfm?id=27&t=4 More stops for gas might benefit fuel vendors by providing them more opportunities to sell you higher-profit impulse or convenience purchases over the counter.