Soot makes the oil black. It has to be related to blowby, the 6.7 has the same issue with oil when the EGR is operational. The piston crown isn't designed for circulation of the air like the 03-04 pistons. The timing and 3rd event "push" the soot into the oil for lack of a better term.. I am not 100% sure how it does it, all I know is that it does it.

The soot is causing the oil to go black, no doubt about it. The leftover oil in the block is enough to blacken the oil instantly. Soot molecules are sub-micron, they are very hard to filter out until the agglomerate. Modern diesel oils fight agglomeration, which is a double edged sword. The soot is harder to filter, but does less damage.

Make sense?

 

We're on the same page with this one, but I don't see how the '04.5 and later engines could have so much more blowby. They have more power, yes, but that would only seem to make a difference in the blowby at the highest power levels. Maybe they have dirtier blowby, but they don't smoke more. Earlier timing could spike the cylinder pressure (diesel knock) more than retarded timing, but they both have retarded timing.

I still can't find a satisfactory answer to the black oil of the later engines. And I can't see how the oil can get black with very very low soot levels in the oil.