Originally Posted by Shortsguy1

There are lots of interesting studies out there that try to address Brian's rhetorical question. For instance, Google Scholar came up with the document below. Full copies are not available for free unfortunately. Two highlights from the abstract which many here will hopefully appreciate: 1) EGR accelerates wear on diesel engines and 2) wear increases proportionally to soot loading in the oil.

Sato, H., Tokuoka, N., Yamamoto, H., and Sasaki, M., "Study on Wear Mechanism by Soot Contaminated in Engine Oil (First Report: Relation Between Characteristics of Used Oil and Wear)," SAE Technical Paper 1999-01-3573, 1999, doi:10.4271/1999-01-3573.

Abstract:

Increase of soot contaminated in engine oil caused by EGR system accelerates the diesel engine wear, especially in the valve train. Wear of metal is affected by many factors such as concentration and diameter of soot, oil film thickness, oil characteristics, etc. Effects of soot on metal wear were discussed from the point of view of soot concentration, and soot diameter and oil film thickness. Wear test was carried out by using four-ball wear tester. Consequently, it was made clear that wear increases proportionally to soot concentration, and relation between oil film thickness and soot diameter plays very important role in wear mechanism. Further, the surface of wear scar was observed by SEM to discuss effect of soot diameter on wear and existence of abrasive wear by soot and its occurrence conditions were suggested.