The ability to hold the soot in suspension is a major part of the spec for diesel oils. There's a saturation point though - comes sooner on an older/tired or hard-working engine.
To go along with that. Imo, how fast the oil gets black is more of a indication of engine condition, than quality of oil. (To an extent)
If it's getting black right off the bat with every change, you likely have a higher amount of blowby.
Most would be amazed how fast the oil goes black on a heui powerstroke. Thats mainly because the heui system still holds probably a gallon of oil, which which get cycled out on first start-up after changing the oil.
Also engines with egr dirty it pretty quick. Our 10 jetta tdi would get dark in a few hundred miles. Once I switched to lubrication engineers synthetic, after a couple changes it would take longer to get dark. So evidently it was cleaning out the engine.
Have also seen some before and after with people running royal purple. Slowly cleaning out deposits in the valvecover/cylinder head.
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