It's stated in the owner's manual that MMT may cause damage to the vehicle. I.E. injectors, and possibly the catalytic converter and whatnot. MMT is found in a majority of octane boosters, cheap race gas, and even cheap pump gas... MMT does leave a pinkish almost rust looking deposit in the combustion chamber and Mitsu had grounds to deny warranties because of that little exerpt in the owner's guide to not use fuels with MMT... This was also discussed on multiple threads. If branded 91 or higher isn't enough for you, here's a thread you might wanna check out...

http://forums.audiworld.com/s4/msgs/95187.phtml