This is totally false. Diesel is not "dirtier" than gasoline, nor is it "less refined". Especially nowandays, since ULSD has to be severly hydrotreated to eliminate the sulfur. Diesel is SIMPLER to refine than gasoline - gasoline needs catalytically reformed high-octane hydrocarbons that take extra steps to produce, whereas diesel is just a straight fractional distillate.
The oil turns black from soot, which is tiny particles of carbon. Unlike a gasoline engine, which vaporizes a highly volatile fuel before combustion begins, diesels inject the low volatility fuel directly into the cylinder. In a limited amount of time the fuel has to heat, vaporise, and burn. The injection systems operate under high pressures (a few thousand PSI in the GM 6.2 and 6.5 to as much as 30,000psi in the newest common rail engines) and inject the fuel through small orifices in the injectors. This causes it to atomize into very small droplets, but they still have to burn from the outside-in. Hydrocarbons are composed of hydrogen and carbon atoms (chemistry 101), and as the molecules are broken apart the hydrogen will react first, leaving little bits of carbon behind to try and react with whatever oxygen they can find in the short timespan they have during the combustion event. Fortunately, normal diesel operation is very lean - there is always an excess of oxygen, so when the carbon does oxidize it does so completely. As a result, diesels generate almost no carbon monoxide. However, there is always a small amount of carbon that doesn't oxidize at all. It forms tiny aggregates called diesel particulate matter, or "soot". Most soot goes out the pipe - the particles are so small they are invisible. When you see black smoke come out of a diesel exhaust pipe that is massive amounts of soot caused by excessive fueling (the engine is operating too rich, which is termed the "smoke limit" - operation in that realm is avoided by the manufacturers for obvious emissions reasons, but enthusiasts will sometimes overfuel their engines to make extra power for short periods; EGT's go through the roof when the engine is "rolling coal", so you risk a meltdown if you do it for very long), but during normal operation the soot quantity is far less and composed of much smaller particles. Newer diesels with DPF's (diesel particulate filters) catch virtually all the soot, so they emit about the same amount out the tailpipe as a gas engine (yes, gas engines make soot, just far FAR less). Anyway, all engines have a small amount of blowby, even when in perfect working order, so some of this soot goes past the rings and into the oil. Soot is an excellent dye material (think lampblack), so it doesn't take much to make the oil black.
Older diesels like your 6.x and my mechanically injected Cummins are actually much easier on the oil than newer engines with EGR. EGR stands for Exhaust Gas Recirculation. Gas engines have used EGR for decades to reduce NOx emissions (nitrogen oxides, which make nitric acid and smog - NOx is only generated at very high temperatures, so EGR works by acting as a neutral gas to dilute the intake charge, which causes combustion temps to be below the point where NOx forms). Anyway, since EGR pipes some exhaust gas back into the intake, some soot (the majority of which normally goes out the tailpipe) gets recirculated BACK into the cylinder. Worse yet, cooler combustion temps make it harder for free carbon to react with available oxygen, and since EGR displaces some intake air there is less free oxygen to react as well. So, the combustion event generates more soot than a non-EGR diesel. The result is that these newer engines generate MUCH higher soot loads than older engines. That was the reason for the rapid progression from CI-4 to CI-4+ and then to CJ-4 spec diesel oils; the later grades have way more detergents to keep all the extra soot in suspension and prevent it from forming sludge. Since our older engines make less soot, these newer oils keep em really clean inside despite the oil itself getting black. In other words, the black oil is a good thing - it means the soot is in suspension rather than being deposited inside the engine
Wow, that was a heck of a diatribe, eh?
