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Most of the oil change opinions out there are based on wisdom handed down from uncle Charlie and his auto mechanic cheerleaders -- the "it's cheap insurance" crowd that gets a new boat every year out of the deal. Getting hard unbiased data is hard.
There was an article in Consumer Reports about 15 years ago on car maintenance. I don't have it any more and can't remember the exact year it came out, but what I do remember is this. They used a fleet of NYC taxicabs to test various drain intervals on standard motor oil. All of the car engines were disassembled at the beginning and at the end of the test, with the internal components measured for the amount of wear. As I remember it, the cars were driven about 100,000 miles. They found wear in the cars that had drain intervals longer than 10,000 miles, but little or no wear in the cars with 10,000 or less. Their conclusion was that the old 3000 mile interval was a pile of %&#@. They recommended 5,000 as a general rule just for peace of mind ("cheap insurance" again), but said they saw little or no engine wear on the 10,000 mile cars and therefore no evidence that 10k did any harm. The 15,000 mile interval cars were starting to show their age, however. Remember that this was with regular motor oil, not the synthetic kind, and the cars were big crude American things with Jurassic-era engines.
Currently, I'm in a 2003 S60 and running on Castrol Syntec. I follow the recommendation of 7500 miles to protect my warranty rights, but I noticed that some European Volvo owners manuals recommend 10,000 miles (16000km) for the same car -- and that's on regular oil. (Maybe Euro oil is formulated differently.) In any case, after the warranty period, I'll extend to something more sensible like 10 to 12k and use Mobil 1.
Finally, I hesitate to mention it because it's only one man's experience with one car, but I drove a 1983 VW GTi for 11 years. I used Synthoil 100, a synthetic that is no longer available and was advertised and priced as a better grade oil than Mobil 1 -- like $9 a quart. I changed oil at 20,000 mile intervals, and once went (gasp!) 30,000. Filters were changed every 5k and oil topped up. I traded the car after 140,000 miles because of rust and safety issues. At that point it was still getting 30-33 mpg, burning zero oil, and still blowing the doors off the other econoboxes on the road. I drove the little Rabbit pretty hard, but the engine was clearly ready for another 140,000. I wish the rest of it had held up as well; it was fun to drive.
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