Definitely motor oil. In fact in one occurrence, I went through about 9 quarts of oil in 200 miles, shut her down for a few hours, drove back a few more hours and it didn’t leak a drop!

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'91 350SDL 165K miles
Lovin' her until she blows...
'01 Dodge Ram Cummins 4X4 DWR
Stump pullin' beast...