Which octane gas to use in a c230 Kompressor?

I just purchased this car about a week ago and I really don't know much about them.  The dealer that I purchased it from told me that I just had to run mid grade fuel or 89 octane.  I have also heard that you need to always run alteast 91 octane fuel.  Should I always use a 91 octane fuel?

stick with the higher octane....recommended for a reason, better performance, cleaner burning, 89 or better at least.

Thanks!