Mixing oil weights

Kern Dog said

The story I read is that the MDS system needs the thin 5 w 20 oil to work properly. If your engine does NOT have MDS, 5 w 30 is fine, as is 10 w 40.

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i remember reading a bill jenkins publication where he did a study of oils , on the dyno , he said that at full engine heat , a thick oil showed to be no thicker , or make more or less h.p. than thinner oil to start with ....