MAZDA recommends the use of 95 octane fuel in my 2006 Mazda 6, but I've been using regular 91 unleaded since day one and have done 130,000km. But I recently read that you could damage your engine if you use a fuel with too low an octane rating for a long time. Should I switch to the recommended 95 octane/E10 ethanol fuel?
In a sense you've already answered your own question. You have used 91 octane without a problem. The Mazda 6 will cope with the lower octane but will run at its best on the fuel with the higher number.