the whole 3000 mile oil change was a marketing gimmick to get people to change their oil more frequently. it's not necessarily bad for your car, but you are dumping oil that is still perfectly fine out of your car.
oil change intervals really should depend on your driving habits. if you're putting a lot of miles on your car in a short period of time, towing, driving in high temperatures, or just redlining the crap out of your engine and driving very hard, you should change more frequently. plus, our cars are turbocharged, and i believe our turbos are oil cooled, so there is more demand for good oil in our cars than naturally aspirated engines. for those of you who like to drag race at every stoplight, and gun it as fast as you can every time you're on the highway, you should consider changing your oil around 5,000-7,500 miles with a high quality synthetic oil. 7,500-10,000 if you're a normal driver that likes to get on it from time to time, or 10,000-15,000 if you're a grandma that likes to get groceries every now and then.
you can send your oil to a lab to have it analyzed. that's the only way you can really tell if the viscosity has broken down. the dark color really doesn't mean the oil is bad.
if you have a higher mileage car, you eventually develop cracks or blemishes in the cylinder walls. these get filled in with oil and other small deposits, which "seal" them back up. if you change your oil too frequently, these seals will basically flush out, and you can prematurely wear the piston seals by the way they rub against the cracks and blemishes on the cylinder walls.
hundreds of tests have been done between different types of high quality synthetic fluids, and frankly, I don't know what study to believe anymore. they're all good: motul, castrol syn, lucas oil, redline, mobil 1, royal purple. all of those are great, and will be way more than enough to get the job done for a majority of drivers out there. if you track your car frequenly, it's a very different story. there is a much higher demand for better oil, and quicker change intervals.
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