Octane ratings on the pumps today reflect the AKI (Anti Knock Index) which is the average of the old RON and newer MON indexes. To get to the point, your best bet is to use the fuel recommended by the engine make, so long as that fuel does not cause premature detonation, sometimes called "ping" or "knock." That easily discernable sound (turn the stereo off, please) while accelerating is the result of fuel being compressed and combusting before it reaches the top of the stroke and before it reaches TDC. The early combustion can result in damage, although not immediately, to a street car engine.

You can use the lowest octane that doesnt result in ping or knock. Temperature and humidity can cause an engine to require higher octane as well. Hot days with low humidity, like in the Southwest, can cause some cars to need the next higher level of fuel. Except for a measley 1-2 HP, there is no power gain for using higher octane fuel in our fuel injected cars. You get more horsepower through more air and fuel, not octane in particular.