Hi, I personally use Premium unleaded 93 octane on both of my BMW’s and wouldn’t use anything less. However the users manual for the 2000 X5’s found below states that:
http://www.bmwmotorsports.org/pdf/BM...20-%202000.pdf
“The engine uses lead-free gasoline only. However, you can refuel with different fuel qualities since the engine is equipped with knock sensors. In order to achieve rated values for engine performance and fuel consumption: > Super lead-free premium gasoline (91 AKI). AKI = Anti Knock Index The minimum fuel quality is: > Regular lead-free gasoline (87 AKI). Because of the engine’s design, you should refuel with this gasoline only as an exception.”