I know the MINI requires 91 octane fuel. However, my father just bought an Infinity M45, and the manual says "use 87 or 91 AKI fuel - or as low as 85 AKI in Colorado or other states above 4000'"

Is this true for the MINI? I know it's only 20 cents a gallon but thought I'd ask anyway.

 

You can simply try a tank-full of the lower octane fuel in your MINI and see how well it works. If it doesn't knock or appear to lose performance, just keep using it! In fact, the performance might even improve!

 

Higher altitude=less air pressure=less actual compression=less power. The engine will regain some power though its computer timing system but essentially, you can use lower octane cuz it will also retard the timing (less air pressure=different fuel/air mixture).
Main reason to use Premium in my book is the amount of detergent additive in it.