Berto they don't all use oil - i remember when i first got mine with 66km (about 40k miles) i never topped it up between 15k services there would be about 3.5 to 4l in the pan. Mine only started using oil at about 300km.
i agree with this, IF the oil is changed regularly they don't burn oil until they reach higher mileage's.
had mine from 10,000 miles at 6 months old and it had had 1 oil change before i got her.
i used 10w40 semi synthetic oil and changed it every 5,000 miles, got to 117,000 before engine was striped for power gains (flowed head) and only using 0.5L oil in 5,000 miles.
only measurable engine wear was on no4 bore, it was close to tolerance for ovallity mid stroke. still under but close. no noise from timing chains or lifters and she was running DBilas cams at that time (bad choice
)
yes what your saying is right, if manufactures had made the pistons better, spent more money on them they would last longer before needing overhauling.
but also if the owners cared for their cars more they would last even longer still.
20,000 miles is way to long to leave the oil in for most peoples cars, engines that don't get cold that are run for long hours each start up can go longer on oil changes. your average shopping cart car how ever goes through many hot cold cycles between oil changes which breaks down the oil faster than those engines that stay hot for longer.
i used to work for a bus company and the Volvo B10M engines would see 500-750,000 miles before needing overhaul with 20,000 mile 3-4 weekly service interval on semi synthetic 15w40. these buses did 20-22 hours a day with out being switched off, just for a turn around nightly inspection and clean. they would still have minimal oil burn.