<<People can do as they like. However, IMHO extending drain intervals on a vehicle under warranty is not a good idea.>>
OldCowboy, this old non-cowboy fully agrees with you. I go even one step further and believe extended drain intervals, and by that I mean significantly longer than the vehicle manufacturers recommendation, are not a good idea in vehicles after the warranties expire either.
The basis for that belief is largely based on the opinion of a friend and riding buddy, now deceased, who was a research petroleum engineer for one of the largest US oil companies, and the most knowledgable person I've ever know about motor oils. Harry was very non-dogmatic in answering the many oil related questions he was invariably subjected to on rides, including never claiming his companies oils were better than other companies. In fact, the only issue I recall him really being really dogmatic about was oil change intervals and he strongly recommended following the vehicle manufacturers recommendations, even if synthetic oil was being used. His reason for that is the same as has been stated by someone earlier in this thread, the accumulation of contaminants in motor oil with increasing mileage and that synthetic oils can't avoid this, or at lease couldn't at that time.
For me this raises the question of what has Amsoil done to their synthetic oils to have extended their drain recommendations? In other words, how has Amsoil modified and improved their oil to deal with the increasing contaminants as mileage accumulates? Have they found a way of neutralizing the contaminants (I can't conceive of any way to eliminate them without changing the oil) that other companies have not?
Perhaps such has happened and I would be very interested in more information on this. Meanwhile, until I see some independent research (and I emphasize independent) that proves extended drain intervals work as advertised, I intend to follow the manufacturers guidelines as I feel the people who design and manufacture the motors are in the best and most objective position to know what's best.
I have an amusing anecdotal story about a person using what must have been one of the longest drain intervals in history but it's long and will have to wait for another time.
Mike
www.rtwrider.net