The dream van (2004 E350 5.4) now has 666,958 miles on it and the engine and trans are both original. In fact the trans fluid and filter is original. Change engine oil somewhere between 100 and 200 hrs. By the way the engine has 14,397 hrs on it. Engine has always used a quart of oil about every 2000 to 2500 miles or 40 to 50 hrs. Use Penzoil and genuine Motorcraft filters. Air filter once a year. Replaced the water pump at approx 500,000 just for safety sake. Changed antifreeze then. Did have three COPs (coil on plugs) fail at the same time (630,000 miles). Came off trip running good. Did a routine service and then the next time we started out it just wouldn't run above 30 MPH. Codes said 6, 7, 8, cylinders weren't firing. Replaced the three COPs and it has been running good ever since.
Good Luck with yours! Dieseldoctor