[Stang70Fastback]

So I just drove my new BRZ off the lot. The salesman filled it up with a tank of gas (it was only at 1/4 tank.) I know for a fact that he put 89 octane in it. I then drove it 90 minutes home. I'm now reading the owner's manual and it says never to use less than 91, and there are lots of posts on the forums about how 89 will cause damage.

This is a brand new car (had less than 10 miles on it.) Is 89 octane "safe" in the sense that you just don't want to do it long term, but that the car will just adjust timing and run poorly until I use up this tank, or are there larger forces at play here and should I be doing something else...? Will it cause even MORE problems due to the fact that I'm breaking the engine in with the wrong octane? I'm really hoping it's one of those things that's really not a big deal and that I shouldn't worry about causing damage.