91 will not hurt the motor. It will have slightly less power but you wouldn't be able to feel it. You may notice the engine running smoother and quieter is all.
Higher octane burns slower and cooler and in car engines and allows for more timing advance, usually around 5 degrees or so faster.
From Genesis: "And God promised men that good and obedient wives would be found in all corners of the earth."
Then He made the earth round...and He laughed and laughed and laughed!