Unless you've increased the compression or altered the ignition timing, stick with the mid-grade called for in the book. At best, there is no advantage to using more octane than needed. Since higher octane lights off slower, it could cost you a slight bit of power and increase carbon buildup.
BUT, if you have to choose between the expensive stuff or low octane camel pee at a 2 pump station, go with the good stuff, as too high octane rating won't cause the problems that too low will.
Don't try mixing high and low in hope of getting mid grade. Unless you are a petro-chemical engineer, there is no way to know what proportion is correct because the chemical aditives may not be the same.
