usually for Honda l4, 86 regular is good enough, but i think for S2000, you need 91.
but for V6 or higher, it's usually 91 or higher
but it also depeonds on if you have turbo or not
for most of the turbo/supercharger, you need at least 91 or even 94 or higher, higher the gas octane, smoother the fuel flow will be going through fuel injector, therefore, the boost will be smoother and won't get lag from unwanted material from low octane gas.
Correct me if i'm wrong.