You're supposed to use ultra-low-sulfur diesel fuel. 2007-model on-road diesel vehicles have to meet tighter emissions standards than previous diesels had to meet. A key to meeting these standards is reducing greatly the amount of sulfur in diesel fuel. Without so much sulfur, diesels can use catalytic convertors and particulate (soot) filters which high levels of sulfur would ruin. Until this year, diesel fuel sold in the U.S. was among the worst in the world. But, this fall, ultra-low-sulfur diesel (ULSD) will hit the pumps. All on-road diesel engines can use ULSD with no problems.

Trouble is, the new sulfur standard is so low that it doesn't take much "low-sulfur" fuel to push the sulfur specification past the point where it could cause damage to these new emissions systems. Given that gasoline, "regular" diesel, and ULSD all travel the same petroleum pipelines and tanker trucks and ULSD will go into not-quite-empty diesel tanks at fuel stations, it will take a bit for fills of ULSD to sufficiently dilute the existing low-sulfur fuel. Conventional wisdom in the diesel-user's world is that three "turns" of a tank should bring the sulfur level down low enough. But, in the meantime, if you have a vehicle that requires ULSD, the warning reminds you that the fuel you're thinking of pumping doesn't qualify as ULSD yet.

It's kind of like when catalytic converters were first introduced in cars, making unleaded fuel necessary -- the warnings about the old fuel containing lead were there to warn you. In very short order, just about every diesel pump will have ULSD and we'll all be happier.

(Sorry if this turned into a book. I tried to edit...)