I had a 2016 F700GS and I used 89octane (mid Grade) without any issues. I now have a R1200GS and at first I used the recommended 91 or better fuel. I have switched to 89 and have noticed a slight drop in power but almost negligible. Is this doing harm to the engine? The Manuel calls for 89 as a minimum. The difference in price here in NYC can be as much as 30cents a gallon.