I bought my 05 800 new and have used premium a couple times, and didnt notice any gains. Always use the cheaper gas, price wise, and at 96,650 miles still running strong, engine never been apart.Several things can determine if you need higher octane besides compression. Combustion chamber design probably is the bigggest factor. The higher octane fuels burn slower, so the piston travels farther past TDC before the fuel is completely burnt, unless you start the combustion sooner, advance timing. I beleive the more complete the combustion is while the piston is at TDC, the more torque you create, thus you need less throtle to get the same exceleration, increasing fuel milagage and you get more power when you do crank the throttle. This is just my opion on the power and milage. Makes sence to me.