As well as my 09 TCH I have used synthetic after break in - (I do it at first oil change) - on my last 4 cars and 3 motorcycles. For warranty purposes I guess you have to adhere to the mileage or close to it but the kind of chemical degradation and breakdown of molecule sizes just does not occur with synthetic. It is cheap insurance in my view if you want to keep the engine running with little wear for a very long time.