My very firat lesson on diesels was on the oil chage, some 30 years ago.
This was on a 220D 72, I bought the car and drove straight to the service shop for some essential maintenace including oil change, picked up the car after service, it was late friday afternoon. When I got home everyone was out to see my new car, popped the hood to show the motor, one of my buddies pulled the oil dipstick out, my god that SOB did not change the oil but charged me for it, it was pure black, I was pissed.
I was not going to drive that car with the old oil in it, so I went ahead and changed the oil and filter, after 5 minutes of idling the engine I pulled out the dipstick, well, lets say I was a little embarrased for what I saw, pure black.

Vahe
240D/77 350K