I was talking with a friend of mine today who has been driving big rigs the last year.(For Werner--one of the largest trucking companies around) He told me that the oil change interval on his diesel tractor trailer was between 3200-3600 gallons of fuel which equated to between 20,000-25,000 miles. This high mileage change was not from neglect but was specified by the mfg. They usually get about 500,000 miles from an engine.
Why is there such a long period between oil changes in these vehicles which see extreme duty and in autos the oil change interval is much shorter? (By the way, they use 15-40w oils in the trucks)