The sludge is the semisolid leftovers (also known as "goo", "gunk", and "crud") from used engine oil that sticks in piston cylinders and can reduce performance and even damage engines to the point where replacement is required. VW's main offender is its 1.8 liter turbocharged four cylinder found in 1997-2004 models of Jetta, Golf and Passat, as well as in the Audi A4. Volkswagen is in the process of alerting some 400,000 owners to the potential problem. Then we have Chrysler— they're extending warranties on their workhorse 2.7 liter V6, which powered many popular minivans from 1998 to 2002. Toyota suffered under a similar problem two years ago, and had to extend warranties and supply new engines when necessary.