My Lady is a 87 model D21, with a SD-25 Diesel, and I changed my oil yesterday,
I fitted "Penrite 20-60 as recommended by Penrite to me for Her age and the
Moderate Speeds we travel at, She idles at twenty PSI and runs with the slower traffic at 46 PSI oil Pressure, and burns no oil at all between Oil Changes!
Hope this might be of assistance to you and others,
Merry Xmas to you all!