Food for thought.
You may not do damage by using lower octane fuel. But most likelly you will spend more in the end. With lower octane, the engine is more likely to pre detonate. The knock sensor will pick that up and compensate for it. It does this in 2 ways. 1 would be to retard ignition timing, the other would be to richen the fuel mixture some. It does this in trying to lower combustion temp down. So imo the end by produuct if this would be less engine power but more fuel used. With less engine power, equals less mpg equals more money spent.