1) Mercedes' new 50-state legal 3.0-liter turbodiesel V-6 engine, which goes under the trade name BlueTEC in its E-Class, R-Class, ML-Class, and GL-Class vehicles, does not need urea to run. Period.
2) The new BlueTECs do need urea to meet U.S. emissions standards, and as such M-B brokered a deal with the EPA to ensure that if the seven-gallon urea tank goes empty (seven gallons are expected to last roughly 10,000 miles), the vehicle's emissions systems will only allow 20 additional starts until your Benzo says, "No mas!" So, supposing you've completely ignored the array of warnings, lights, and readouts telling you to head to your local M-B dealer to get the urea tank refilled, yes, you'll get exactly 20 more starts and not one more.
3) We suppose, if you're ultra-stubborn, you can do what schoolbus drivers in Detroit and Minneapolis do in January, and leave your Benz diesels idling all night long (in the case of the former, so the old low-tech bus diesels won't refuse to crank on the bitterest of mornings). You could, indeed, save those 20 starts for 19 oil changes, and just leave the engine running interminably. Diesels are ultra-efficient at idle, where they also don't produce a big thermal load on the cooling system, so she'd purr happily away while you sleep. And you'd never have to change the urea for, say, 200,000 miles. Ha ha! Take that, Uncle Sam and the EPA!
Or, simply, having decided to purchase one of Mercedes-Benz's new efficient, torque-rich, and extremely clean BlueTEC diesels, and enjoying highway milage in the mid-to-high 20s in your 7-passenger, 5,000-pound-plus GL320 BlueTEC SUV bruiser, or mileage solidly in the 30s on your posh E320 BlueTEC, you could have the service techs top off the urea every time you get the oil changed. I'm just sayin'. Seven gallons in 10,000 miles sounds a small price to pay for ultra-clean emissions and at a reported cost of $7.50 per gallon for the urea, it doesn't sound like a cost that's out of bounds for the average Mercedes owner.
Diesel has an uphill battle here in the Unite States, but with our open ear and open minds, perhaps U.S. drivers will open their driveways again to the notion of, say, a VW Jetta that can get 58.85 mpg in the right hands. Diesel is not THE answer, of course. But I think it's one small part of the solution, and I hope it gets a fair shake this time. Diesel got killed the last go-round by the clap-trap GM Olds 350 diesel. Let's hope it doesn't get killed this time around by misinformation and knee-jerk headlines.
I'm Colin Mathews, and I approve of this message. Haters, get your pens and keyboard ready.--Colin Mathews