Octane requirements are based on piston compression ratios. If you run too low an octane the fuel can detonate before it is supposed to and your engine doesn't run quite right (you'll get less power and you hear a "pinging" noise). At the least, run a few tanks of 89 and see if you notice a difference.
Avoid predictable disaster caused by unpredictable events, keep yourself open to positive outcomes from improbable events. -Aaron C. Brown