I've got a 5 gallon can sitting in my Dad's shop right now that I got from Sherwin Williams for ~$30.
And I'm down to the 1/4 tank I wanted to be at before adding a gallon of toluene to the tank.
For a grand prix with an 18 gallon tank, already using 93 octane gas, the equation for the toluene is;
(558+7x)/6=y where X is gallons of toluene and Y is final octane rating.
Note that this equation will ONLY work with 93 octance gas in an 18 gallon tank. If you have a different car, you will need a different formula.
I'm going to go to the gas station now and we'll see what happens... BBS