They probably said 91 because that's the highest you can get in many parts of the country. I live in Colorado and 91 is "premium". I always run that in my X, as I noticed a bit of a power drop when running 89 (mid grade here). But I have a 3.0, so I'll take all the power I can get Running mid grade shouldn't be damaging at all, but as others have said, just try a little bit and if you notice a serious drop in power or MPG, you know not to use it.