A clean engine is a happy one. I change the oil when it starts to get kind of scuzzy. On my 1966 250, it is usually at 3,000-4,000 miles with conventional motor oil. Also change the filter when changing the oil, because why leave a quart of cruddy oil in the system.
Make sure you take your used oil to the recycling center. That way no one can say you are wasting oil.
Doug 57.
