Mixing Sunoco SS 100 to 91 Octane-Any Negatives Besides Price?

Go to the local small airport and get 100 Av Gas. Most likely cheaper. 110 and 100 were both 10 bucks a gallon at Laguna Seca last week. A buddy that I help run a E36 BMW in Lucky Dog endurance racing has the thing tuned for 60/40 mix of 91/100 Av gas. Higher the octane the slow the gas burns thus reducing knock...