To be clear, you must use Ultra Low Sulfur Diesel (ULSD), which is usually available. This is not to be confused with Low Sulfur Diesel (LSD) which is still available at some stations and is not supposed to be used in the X5. This will all be moot by 2010 when only ULSD will be sold, but until then, you need to make sure that what you're putting in the tank is ULSD. Stations are required by federal law to label diesel, but I've found this to be hit or miss in reality.