Improving the efficiency of internal combustion engines is important for reducing global greenhouse gas emissions; the efficiency of spark ignition (SI) engines is limited by the knock phenomenon. As opposed to naturally aspirated engines, turbocharged engines operate at beyond research octane number (RON) conditions, and fuel octane sensitivity (OS = RON – motor octane number (MON)) becomes important under such conditions. Previous work by this group [ Energy Fuels 2017, 31, 1945−1960, DOI: 10.1021/acs.energyfuels.6b02659] elucidated the chemical kinetic origins of OS; this study is extended to provide a qualitative, as well as quantitative, definition of OS, based on fundamental ignition markers. A varying amount of toluene is blended with various primary reference fuels to match the ignition delay of the targeted research octane number fuels, allowing a range of octane sensitivities for each research octane number. This study establishes a correlation between OS and heat release rates at low, intermediate, and high temperatures. The significance and chemical origins of intermediate-temperature heat release in defining the OS of toluene blended in a mixture of iso-octane and n-heptane is also clarified. For the toluene–iso-octane–n-heptane mixtures considered here, low-temperature reactivity was not found to be a key marker of OS. The results also show areas of improved efficiency in beyond RON operating conditions, where high-sensitivity fuels could be beneficial.