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I would stick with the recommended 5w30 as per the manual. Thinner oils tend to be better for FE, but there is a trade-off in terms of engine wear. Less than 1 % in FE savings is not enough to make me switch, especially considering it is a vital element to engine life.
My Echo has only seen 5w30 mineral oil since day one. I might switch to synthetic in the long run though.
Hope this helps.
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