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  1. Synthetic oil in a 4b boat motor: Rotella T6 5w40?

    General Discussion
    I am thinking of switching from 15W40 petroleum based oil (Chevron) to synthetic oil....possibly Rotella T6, 5w40. Temps are typically 20F in the winter, up to 75F in the summer. Is this a reasonable oil for these motors? What are other folks using? Thanks in advance.