Putting premium gasoline (91 octane or higher) in a car designed for regular won't harm the engine. The engine's computer will simply adjust the ignition timing and fuel mixture to account for the higher octane. You won't get any noticeable performance increase; the higher octane fuel is only beneficial if your engine's compression ratio requires it to prevent knocking (pre-ignition). You'll just be paying more for fuel that's essentially wasted.
In short: No harm, but no benefit either. Stick to regular unleaded unless your owner's manual explicitly states otherwise.
