15,000 miles.

The 15,000 mile average is correct...but... Actually, that is the recommended oil change interval on some engines. My Cummins ISX 15-litre engine is recommended to go for 25,000 miles between changes. Others have other recommendations, but there's a caveat in the soup, so to speak. You should have the oil tested, and change it according to how your tests come out. When we worked for one of the really large carriers, they used an entirely different system. They changed oil according to how many gallons of fuel they used. This is really a pretty good system, because it reflects real-world work done by the engine, rather than an arbitrary mileage. On trucks driven by a solo driver, they might reach their limit, which was 3100 gallons the last I knew, at as little as 12,000 miles. On a team truck, that idled very little, it might be as much as 21,000 miles. Now, the real question is, does extending the drain period make sense? Look at it this way. If you can run an engine at a long drain interval, and still have good oil performance, you can save an immense amount of money. Just as an example, the company I mentioned about owns over 8,000 trucks. It's hard to calculate the exact savings, but considering that each oil change uses almost 12 gallons of oil, extending that oil change interval only 10% would save (by the calculations I did at the time I worked there), some 5,000 gallons of oil per day. That is used, dirty, contaminated oil that had to be disposed of. That's a whole tanker truck of oil every day. At one time, they double-filtered it and added back into their diesel fuel, but the EPA really doesn't like that any more.