Europe uses ROM, which tends to be higher than MON. I have seen the archaic test equipment that some refineries use based how octane ratings started over a century ago . It is a large one cylinder engine that is run at idle until it knocks under compression and then the fuel is calibrated. Some labs now use a spectrometer test method rating the iso-ocatane and heptane in fuel. Then there is the higher octane rating of blending ethanol due to its cooler burn and all the chemistry gets insanely boring. Bottom line, use what makes your car run well as per its tuning without detonation and/or damaging the plugs.
The octane rating for aviation fuel is different. The pilots here can tell you about it. And so is diesel.