Have run 1960's DD 8-71 and newer 1995 DD Series 50 and with the std modern oil filter the oil is black on restart after a change.
Have a 6-71 built to WWII specks with a bypass filter , called color filter in that era, that is clean at 5 hours and just going light tan at 100-125 on change.
The black is called soot , but weather it is harmless or functions as fine grinding compound is unknown.
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