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Hmm, it's all conditional. Go here for an interesting .pdf EPA

web page on 5w-20 This site is Korean, and contains, evidently, a paper on the development and testing of modern 5w-20 oils. The text is unreadible, but the charts are in English and contain some interesting test reports of 5w-20 vs. 5w-30 using improved (group 2?) base oils and additives. I...

My guess is that whether a 5w-20 works as well as a 5w-30 is engine specific. Ford & Honda certified only certain, recent engines for the thinner weight. It would be interesting to compare 5w-20 vs. 5w-30 across those engines.