six and a half a dozen the other... if it's worded in a way to interpet 91 or above as 91 or above than I doubt you'll be harming the engine.Premium unleaded fuel 91 [ (R+M)/2 method] or above (96 RON or above)
This vehicle is designed for and requires use of premium unleaded fuel. If 91 octane fuel [(R+M)/2 method] (96RON) is not available, gasoline as low as 87 octane [ (R+M)/2 method] (91RON) can be used temporarily for emergency purposes. Use of gasoline lower than 91 octane [ (R+M)/2 method] (96RON) can decrease performance during its use.
Refueling the vehicle with the correct octane fuel a couple of times will restore vehicle performance.
Guess the only way to know is to tear our engines down in a few 100,000k's