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I don't know of any entity that has more experience and has done more research and empirical testing than the OEMs. It is in their best interests to recommend change intervals that maximize engine life yet also maximize oil life. If anything their recs are conservative as they have to account for worst case scenarios. Thus, I generally go by what they recommend.
Shops service cars to make money. Do the math.
Guys often like to think they are smarter than OEMs or anyone else. Many of them actually brag about changing out honey-colored oil ahead of it's time.
Or maybe they learned from Dad, Grandad, or Great-Grandad to change every 3k cuz that's what everybody did back in the day.
Then on the other hand I've heard several instances of people that seldom or even NEVER get the oil changed, and they rack up many thousands of miles with no incident! Well I'm not willing to go that far but it is safe to say there is a lot of premature oil changing going on.
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