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I thought I'd just post this up because a while back i had the same question and I've heard ridiculous claims before.
Mixing different octane leveled fuels normally causes no damage. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. Lower-octane gas can handle the least amount of compression before igniting.
The only reason there would be a problem is if your car either calls for specifically a higher or lower octane.
Mixing the 2 simply lowers the level so if your using 87 and mix it with 93 your getting something around 90 or 91. If thats what your going for hope you have the money to spend but you could also go get an octane booster at Canadian Tire or an automotive store.