The aromatics toluene and xylene have extremely high octane and high btu (~115). These are sold as paint thinners. You can mix either with gas to raise the octane without changing the chacteristics of the gas too much. Aromatics are already in gas so not much change except octane. If you have a choice use toluene for no other reason than that is what the old turbo F1 cars used to use for such high boost and HP.

Tom Graham