royal purple is good ****.
bmw's by design comsume a certain amount of oil. it is used to lubricate the valve train. if it was just low enough to set the light off, top it up.
if you have the proper oil level it could be a few things. bad oil level sensor, moisture contamination, or if you take short trips the oil will get aerated and throw the sensor off. etc etc.
just keep an eye on it with the dipstick, its work since the begining of time, and is better habit then relying on the oil light.