California actually has different gas that produces fewer emissions - it's part of the reason gas is more expensive here. It's CARBOB blend instead of RBOB/CBOB.

Today I learned, thanks!

TIL, thanks for the info

A have a 2007 European car that wants 93 according to the manual, but will "run acceptably" on 91, not sure if it's changed in recent years though.

EU octane numbers are different from US octane numbers

I'm in Europe and here it would be "98 recommended, 95 allowed with possible degradation in performance", as it's the case for my 2006 car. More modern small volume turbocharged engines should be even more sensitive to octane number due to higher compression ratios than common American engines of twice the volume for the same horsepower. The engine ECU will reduce boost pressure and fuel charge based on feedback from the knock sensor.

Roughly, 98 RON (Europe) = 93 (R+M)/2 (USA)

They are, however, manuals and documentation for US market vehicles will be specified in AKI (aka '(R+M)/2') regardless of who manufactured them. Some manuals also specify both, and that can be a source of confusion.

Octane requirements are also a function of elevation