As diesels got more efficient, they got worse at idling for prolonged periods. Cummins has a TSB about prolonged idling on their later Direct Injected engines causing stuck valves, increased wear on ringlands and increased fuel contamination of the oil.
Why? because the more efficient the combustion process, the less waste heat is given off. At low
RPM and loads, EGTs will get below 300 degrees, this means some fuel goes unburnt, and that works its way around the rings and into the oil. That's Cummins' interpretation anyways.
(I worked a contract developing standalone engine management systems, and got to spend time working with a gov't contractor on a scratch-built Diesel engine management system based on common rail injection - you should see what happens when buggy software sticks open an injector with 26,000psi fuel pressure in the rail! you have to empty out the whole room!)