I heard that using unleaded gas in a 2004 X5 is for good performance. If use any other gas, will that damage the engine?

factory built engine for certain type of gas...if u use leaded it will clog up cat converter...low octane will run in car made for premium but will have low performance/mileage and will build deposits....use what manual calls for...

The car would have come from the factory with a recommended octane. I recommend you stay at that level or above. Example, if the car has a minimum octane rating of 89 then try a 91 or even a 93 octane. Using a lower grade can hurt the engine.