TOM: Aside from the potential inconvenience of that, Mercedes estimates the expense at about $1,500 for every 100,000 miles you drive. RAY: Of course, like everything else you do at a Mercedes dealership, you'll pay more to get your urea topped up there. So if the expense is a concern, you might look around for an independent shop that specializes in German cars. You'll pay less of a premium there. TOM: You can buy your own urea (under the Mercedes brand name "AdBlue," or as another brand) and learn how to add it yourself, if you really want to cut your expenses. RAY: Isn't it a good thing you're saving all that money with your high-mileage diesel, Dan? Otherwise, how would you be able to afford all that urea? Which is cheaper, buying or leasing? Should you keep a car forever or dump it after three years, before trouble starts? Find out in Tom and Ray's pamphlet "Should I Buy, Lease, or Steal My Next Car?" Send $4.75 (check or money order) to Next Car, P.O. Box 536475, Orlando, FL 32853-6475. Get more Click and Clack in their new book, "Ask Click and Clack: Answers from Car Talk." Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com. (c) 2012 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.