I've been reading this site (and simmilar ones) a lot, and I noticed a lot of people (mainly from USA i think) who mention 3000-6000 mile intervals for oil changes.
I'm from Europe, and driving a Korean petrol car, and my service/oil change interval is 20.000km (~12k miles). A lot of diesels have even longer ones (30kkm - 18k mi). These are the intervals specified in original, manufacturer's service booklet.
Why are the numbers so different?