Hello Eric. This post really highlights to your other post re: what oil to use in a diesel. Because a diesel has higher compression than a gas motor, a larger percentage of fuel & combustion contaminants passby the piston rings & end up in the oil of a diesel vs gas motors. Thus your oil "Blackens" at a quicker rate. Also, the oil is asked to carry this heavier load of contaminants in suspension and still maintain it's lubrication & viscosoity levels. That is why several of us recommended Delo 400 as it is blended to maintain viscosity, is Highly Detergent also, & maintain it's lubrication capabilities while up against the fuel dilution from diesel engines.
Richard