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Well I read from various service books that the original specs on these engines called for a 20w50 so when I first got into these I just went with what was stated... However I have been wondering of late why such a thick oil is needed hence the drop to Rotella 15w40.


NEVER seen a manual that OEM stated to use 20/50 oil. Most of the time they specified 10/30.
Yes big blocks do air lock at times. Have had it happen but it is not all that common.