Always use the RON recommended by the manufacture. This goes for every car not just the SVX. YES the cars computer can retard the ignition by several degrees if/when it senses engine knock. But this feature is basically to be used when premium gas is not available. Basically the anti-knock sensor should not be used as a plan-B to save you several dollars per tankful of fuel. Premium gas also contains numerous chemicals in higher concentrations than regular unleaded that are critical to the efficient long term operation of numerous components and to the overall well being of your engine in the long term.