In the old days, putting low octane fuel in a car designed for premium would result in an audible knock, which was a reminder to the driver not to be penny wise and pound foolish. The ECU in any modern engine will use a knock detector to adjust timing. Using regular fuel will work, but the timing will end up being electronically retarded to avoid knock. The result may be obvious changes to fuel economy and performance, and not so obvious changes in EGT's which will affect long term durability. Emissions tend to remain acceptable, as the ECU uses the O2 sensor to maintain closed loop operation. If the manufacturer recommends premium, that's what you should use.