Eclipse GTS Regular fuel?

It will run on it but with reduced performance . The motor features a knock sensor that reports fuel octane values to the ECU. When a lower octane is used, knock is present and the ECU retards ignition timing to prevent engine damage. The ECU will continually reduce timing until knock values are with in a predetermined safe range but it will only provide a limited range of adjustment. What that range is, I can't tell you because I play with the 4G63. I would limit the time spent at full throttle when running on regular just to be safe.