Here's what an engine designer says.

“The California Air Resources Board and the E.P.A. have been very focused on making sure that catalytic converters perform within 96 percent of their original capability at 100,000 miles,” said Jagadish Sorab, technical leader for engine design at Ford Motor. “Because of this, we needed to reduce the amount of oil being used by the engine to reduce the oil reaching the catalysts.

“Fifteen years ago, piston rings would show perhaps 50 microns of wear over the useful life of a vehicle,” Mr. Sorab said, referring to the engine part responsible for sealing combustion in the cylinder. “Today, it is less than 10 microns. As a benchmark, a human hair is 200 microns thick.

“Materials are much better,” Mr. Sorab continued. “We can use very durable, diamondlike carbon finishes to prevent wear. We have tested our newest breed of EcoBoost engines, in our F-150 pickup, for 250,000 miles. When we tear the engines down, we cannot see any evidence of wear.”

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“Competition is part of it,” said Peter Egan, a former auto mechanic and now editor at large of Road & Track magazine. “Japanese cars kind of upped everyone’s game a bit. With some exceptions, the engines would go a long time without burning oil or having other major problems.”

Hyundai and Kia, the South Korean carmakers, now include 100,000-mile/10-year warranties on their cars’ powertrains. If a relatively abusive driver can count on no major mechanical failures before 100,000 miles, a careful owner can — and does — expect his car to go much farther.

“My parents, with their ’56 Buick, used to have the wheel bearings repacked with grease before a long trip. Nobody does that anymore. The lubricants are better, the machining is better.”

from:
http://www.nytimes.com/2012/03/18/automobiles/as-cars-are-kept-longer-200000-is-new-100000.html?_r=0