Don,

The gentlemen who built the car are not going to profit any from you using a higher price gasoline, but they do say premium only for a reason. They base all the performance numbers off of octane ratings used in their tests, and they only use premium. My guess is that between the engine retarding timing and modifying other parameters to make the itself run smoothly, at the least, you are making less power. Also the lower octane fuels tend to leave more depoits in your motor, something I am not willing to risk. I spend roughly $125 more per year to put in 93 over 87 (based on $0.20 more per gallon at 12,000 miles per year), in my opinion for the cost of one oil change, it is a no brainer, expecially since all the brilliant engineers who designed feel so strongly about the matter....

John