I wouldn't want anything other than BMW coolant in the X5. What I do is keep it readily available. You can buy it on the internet from places like bavauto.com or pelicanparts.com for less than the dealership charges.
Did you get the expansion tank replaced already? This may be a good time to get the rest of the cooling system (belts, hoses, water pump, thermostat, etc.) overhauled if it hasn't been done recently.