With correct oil clearances and an accurate pressure gauge, 15 at idle and 30+ at cruise RPM is excellent. Back when those engines were new, most people used 30 weight in the summer, and switched to 20W-20 in the winter. I'd definitely recommend HD 30 (detergent) oil instead of non-detergent. The non-detergent oils available today are way down in quality, mostly intended for getting a few more miles from worn-out engines that burn or leak so much oil that oil changes aren't necessary. "Fill it up with oil and check the gas!"

If you want to use a multi-grade oil for some reason, 15W-40 Diesel rated oil would be a good choice.
Jerry

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