Did you have it re-tuned or was this done by a previous owner. What was the point of this retune? Racing?
US octane is measured totally differently to the rest of the world so cant be compared by numbers
Octane rating is generally measured as either RON (Research Octane Number) or MON (Motor Octane Number) with slightly varying figures between the two.
Here in NZ, we use RON as our standard rating, the US uses a figure called AKI (Anti Knock Index) which is derived as the average of RON and MON.
In practical terms this means that fuel rated 85-87 Octane in the US is equivalent to 91 here and US rated 93 octane is equivalent to our 98.
Most cars will see no benefit from running an octane rating higher than they were designed for unless they have been modified or re-tuned and in fact it can cause problems with excess carbon build up due to incomplete combustion.
The other one to watch out for are Japanese performance cars from the 90's to early 2000's before flexible fuel mapping was widespread as at the time 'premium' fuel in Japan was ~100 octane RON and many of these cars will have been tuned for it.
My car (2000, Subaru Impreza STi) has to be run on 98 as it has been re-tuned and the mapping setup for 98, this can be a bit of a bother if I go on a long road trip, but generally BP has enough stations along the main highways to avoid too many issues, running a lower octane could result in engine damage.
My bike on the other hand (2014 Hyosung GT650RL) actually likes 91 octane and runs rough on 98.
I had it retuned. A previous owner had put on a boost controller and a blow off valve with no apparent tuning done. Speedtech in Wellington offered me a Dyno tune for basically the cost of replacing/removing those two components anyway.
Gained power, responsiveness and fuel economy so pretty happy overall.