i havent changed my oil since i got my car in oct. 2002.. i havent used my car much... so the oil in my x5 is about 2 years and a half... some said i should change the oil every 2 years regardless of mileage.... i dont know if thats true.. and ive read here that we should change the oil in our bmw's every year... and i believe that diesel engine oils are dirtier than gasoline engines.. and they get dirty very fast....