Actually, I was told by a former mechanic (and current teacher of "power tech" type shop classes for an area high school) that the biggest reason to do a 3,000 mile oil change is because of additives in the oil that start breaking down after that length of time. He said that besides the ability of the oil to lubricate, there are other things involved, like rust inhibitors and detergents put in the oil.
Since then, though, I've also belonged to car owner groups for various vehicles where people claimed they regularly sent off samples of their used motor oil to a test lab for analysis. Typically, they concluded that 3,000 miles is a little earlier than necessary to change their oil -- but sometimes, a 5,000 - 6,000 mile interval seemed appropriate.
I suspect there are actually a lot of factors involved, including the tendency for some engines to get a little bit of fuel into the oil, polluting it and reducing its efficiency. Also, don't forget that in the U.S.A., most drivers really don't do a lot of regular maintenance on their vehicles. The 3,000 mile oil change they go to the quick-lube place for also serves as the interval their tires get refilled with air to the proper pressure and their other fluids get checked and refilled if needed, air filter checked, etc. If they started suggesting people hold off on the oil changes for a lot longer, they'd be driving around for months with low tires, dirty air filters and worn out wiper blades.....
Never been able to understand how the diesel VW engine installed in the Jeep CRD in Europe can go for 9,000 miles between oil changes and yet this daft 3,000 miles thing in the USA persists, for petrol engines. In the UK a Petrol engined car can often go many more miles than 9k between oil changes.
Someone has got it wrong, or right I suppose, depending on who is taking the hard earned money from the driver!
Unless, the quality of the oil in Europe is to a higher standard? We certainly pay a lot more for it, that in the USA.
The truth is it's a rip off! Plain and simple.