I would hazard to guess that the higher the better considering Japan's low grade is typically 93 with the typical high at 98 with 100 avail. When I worked overseas I remember Europe's typical low is 92 and same on the high side as Japan. We get the crap gas at our stations... however much better price.
Personally I only use Sunoco "Ultra 94" unless I am somewhere I cannot find one.
Europe and Asia measure their Octane differently than the US. They use MON. US uses RON + MON / 2 = US octane. Consequently the US octanes will always be lower than the European and Asian values.
Octane: There is NO benefit to gain using an octane higher than suggested by the manufacturer.... UNLESS.
You have re-tuned your engine (chipped, compression change, turbo, etc.)
Here are some examples:
In my 2002 Porsche 911 the recommend octane is 93. In California the best you can get is 91. Consequently the computer retards engine/spark timing to accommodate the 91 octane and I loose ever so slightly some power. In this case it helps to pump in ~6 Gallons of 100 and ~8 gallons of 91 to give me about 93.5 octane and get optimal performance.
Example 2: You drive a honda civic. It has been tuned from the factory to run on 87 octane fuel. You buy an ECU re-flash to get you that extra 10hp. Now the car is tuned to run on 91 octane. You need to run the 91 octane to realize the 10hp and to keep you motor from pinging.
Example 3: you do the same as above but this time you install a turbo with 26 lbs of boost. You need to run the 100 octane straight to keep your engine from detonating early because of the extreme compression you are seeing in your motor.
Essentially octane makes fuel LESS likely to burn allowing more piston compression to occur before the flame front pushes back allowing advanced timing to occur and make more HP.
If your car ECU does not have air/fuel/timing maps in it for fuel with the rated octane, it cannot take advantage of the octane. Some modern cars (I assume the IS-F is included) have sensors (typically dual knock sensors) that allow timing to be advanced just until the edge where knock is detected. This maximizes power. However if your ECU maps do not have info for 95-100 octane, then you will see no benefit.
