timed change or trust assyst?
"If you use an oil that's on THE LIST, you can almost always trust the ASSYST." - Turtle Cochran
The reason you can't
always always trust the ASSYST is because you can actually reset the ASSYST computer to stop using the Oil Quality Sensor and instead cause it to do a straight mileage countdown. If you reset the ASSYST, and then immediately reset it again before driving it, that's what happens. To get it back on the Oil Quality Sensor you have to reset it with the DRB III tool at the next oil change.
As for the oil being black, do a fresh oil and filter change, run the engine for 2 minutes, and then check the oil. It'll be black. There's a chemical reaction that takes place between the oil and the fuel (mainly but not exclusively the sulfur) which causes the oil to turn black. This is normal. It happens with and without an EGR valve. It happens with gasoline engines, as well, only not as quickly.
Unless you are Geordi LA Forge, merely looking at oil on a dipstick won't actually tell you anything about the oil quality.
When I first got mine, for the first, I dunno, half a dozen or so oil changes, I was pulling oil for analysis every 5000 miles and again at the oil change per the ASSYST, and the ASSYST turned out to be spot on with the oil analysis. That's when I felt comfortable in trusting the ASSYST.
I've had the ASSYST tell me to change the oil as quickly as 12,000 miles and as long as 23,000 miles. It all depends on the conditions under which the engine has been operated (temperature, humidity, altitude, RPMs, lots of factors play into it).
Roughly every 7000-9000 miles (usually when the ASSYST says I have 5000 miles left to go on the oil change) I will change out only the oil filter, that way the filter gets changed at about the mid point of the oil change interval.
I currently have a really annoying oil leak that no one can find, without steaming the engine and just looking for it. I have three very highly educated guesses from three very highly experienced Sprinter mechanics as to where it's leaking, all of them highly different guesses. Awesome. But I go through a quart every 4000-5000 miles. Not surprisingly, adding half a quart of fresh oil every 2500 miles really stretches out the ASSYST interval.