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There are a lot of opinions on this subject, but little data.
In 80,000 miles of driving my 2003 S60, I've kept complete data on fuel economy since day one and have used a variety of different octane fuels. I have info on every tank of gas I've burned, including octane, brand, price, and the motor oil in the crankcase. I began keeping this on an Excel spreadsheet so I could answer the same sort of questions you are asking.
My experience is that there's no difference whatsoever in fuel economy delivered by different octanes. 87 had a slight edge by about .1 mpg last time I checked -- not a statistically significant difference, but there it is. Overall, I'm getting 31 mpg over the life of the car in 10% city/90% highway driving in New England.
Oddly enough, I have noticed differences by brand. This may have something to do with additives, but for whatever reason Sunoco, Irving, Shell and Citgo are the winners and Exxon, Mobil, Hess, Gulf, and most of the no-names are losers. I didn't notice a drop when the 10% ethanol mix became universal a few years ago in this area.
The biggest boost to fuel economy comes from slowing down and keeping a steady speed. Next in line is pumping up your tires. I run mine at 38 psi all around.
Things have not affected my MPG: regular or synthetic oil; octane; time of year; snow tires or summer tires (if pumped up to 38psi).
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