I've had my Prius for about 3 weeks and am approaching my first fill-up. In Denver, we have 3 octane choices of gasoline. Regular is 85 and mid-grade is 87. Toyota, like other makers recommend 87 octane. In other cars that I've owned, I still use the 85 and I understand that this is actually the same as using 87 at sea level. With the Prius engine being small, I wonder if this is still ok. People at my dealership seem to disagree (I haven't been able to talk to the Prius Mechanic.) Does anyone have any experience with this?