replacing the oil is a must at 6 years or 100k miles. Obviously the age is going to arrive before the mileage for most Morgan owners. I had such bad 'clunking' after twelve years that I had to get out the AA to ferry my car to the local dealer. They could find nothing wrong but I suspect that if they had replaced the oil instead of just topping it up when servicing the car I would not have had the problems I faced.