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Guys, it's mostly Ford and Honda that spec 0/5W-20 oils ... and I don't think they redesigned a single engine with this thinner oil in mind. All the engines that call for it are older designs with the same clearances as when they called for 5W-30 (like Dad's Mercury Grand Marquis with its 4.6L V8).
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Let us assume you are correct. Why then were these engines specd with 10w-30 to begin with if the bearing clearances are unchanged? I can tell you why the spec was changed to 5w-30: To improve fleet fuel economy to meet govt. standards. As long as the engine outlasts the warranty, it is not the manufacturers problem.