The spec fuel for the Briggs 206 classes will be 87 octane, purchased from the Shell station located at 3300 E. Clear Lake Ave, Springfield, IL.  (This is the same exit that you use to get to I-72 and the track from I-55.)

Be sure to pump at least a gallon of fuel into your vehicle or another can before pumping into your race gas can.  The pumps typically dispense more than 1 grade fuel and there may be some of a different grade in the hose, which could contaminate your race gas.