Dirt and Soot: Both destroy the engine
These samples show that just looking at one aspect of maintenance isn't enough. You have to watch everything.
In this first report we have a Toyota Land Cruiser diesel. We can see that in the first sample (2007) everything is good, with 1 ppm of dirt, 0.06% soot, only 4 ppm of iron wear and 1 ppm of the rest of the wear metals.
By the time it got to the third sample in 2009, with a change in personnel, they were "cleaning" the air filter and ignoring the fuel injection system, causing the ingress of 32 ppm of dirt and creating 4.8% soot in the oil. This resulted in 64 ppm of iron wear, 17 ppm of aluminum and some lead from the bearings.
They changed the air filter, but did nothing to fix the combustion. This lowered the dirt ingress to 6 ppm, but allowed the soot to reach 5.3%. Fortunately they are using a good oil that is resisting thickening, but the engine is probably building sludge in the rocker covers, heads and oil pan.
They changed the air filter and the dirt contamination dropped to 6 ppm, but they lost control of the fuel injection system, allowing the soot to build up to 1.3%, thickening the oil to 19.03 cSt in 250 hours.