I've been changing mine every 5000-6000 miles or yearly. I do use Mobil 1 5w-40 synthetic oil, however. The engine is running fine at 80,000 miles.
Mark
2000 Itasca Suncruiser 35U on a Ford chassis, 80,000 miles
2003 Ford Explorer toad with Ready Brake supplemental brakes,
Ready Brute tow bar, and Demco base plate.