Amazingly it looked like it had been changed routinely every 5,000 miles (30wt oil). I can't explain it other than that I drove it often and in various road conditions in weather 50 to 85 degrees; boulevard cruising, at highway speeds (70-75mph) on extended trips, occasional highway blasts at 80mph, errands around town, etc..
Every 15,000 mile or so I would change out the external oil filter and top off the oil afterwards. Dumb luck, I guess! Or more likely blissful ignorance...