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I've found there to be positives and negatives for both the heavier oil and the lighter. I ran Royal Purple 5w30 for the first couple oil change intervals after purchasing my 80. Downsides were some oil consumption and timing chain rattle on startup. Switched to Rotella 15w40 at the last oil change, have had zero consumption since and no more rattle on startup, but I seem to have lost approx. 1 mpg and the truck feels a bit slower (subjectively, haven't done any real testing I suppose).
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