I only put 95 RON in mine. I've used 98 RON before, but find that while the car seems to have a little more power (maybe it's just my mind justifying the higher price) the car certainly doesn't run as smoothly. I had the same experience with both of my MINI Cooper s's prior to the X5 and our 3 Series. All run rough on idle. Our local dealer recommends using 95.