From the 2004 Service Manual:
"Highly sensitive anti-knock sensors, Refer to Page 8I-6, can distinguish knocking conditions in individual cylinders and retard the ignition timing on the cylinders that are knocking. This anti-knock control prevents damage to the engine and allows operation on lower grade fuel, but only in emergencies. Premium grade fuel is required under normal operating conditions to ensure full power and economy, and because the anti-knock control system may not be able to prevent knocking on low grade fuels under all operating conditions."


Your car will still run on the lower grade gas, but what happens to performance and fuel economy and is it worth risking damage to the engine? I've seen what knock can do and besides if fuel economy drops by one mile per gallon, you will almost break even on the cost per mile driven.