On these old soot collectors the frequency of change is perhaps as important as the oil used. A periodic highway run is needed to get rid of any accumulated condensation in the oil. Prevents sludge formation as well.

You have to get the oil to 212 degrees to boil it off is why. The pertroleum oil does not boil itself till 400 degrees or so I believe.

If you are only doing about three thousand miles a year and changing the oil twice. I would only change the oil filter every second time.

You are probably too young to remember. That one time every thousand miles was the recommended oil change interval on many cars.

At a burn rate of about 1200 to 1400 miles per quart that engine should have a lot of life left. That's if the fuel pressure in the base of the injection pump is still decent. It should be checked at that milage and corrected if needed if it never has been.