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Part of the sludge issue is increased emissions regulations, but part of it, for Toyota at least, is simply because people don't change their oil at the proper time.
There's a big difference between "severe" service and "normal" service. A lot of sludge-sufferers were going for the longer 7,500 mi oil change interval, but did mostly short trips and trips in traffic... which are, yes... SEVERE service. I know a guy who got sludge from missing one too many oil changes.
Follow the shorter "severe" service interval and you have no problems.
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