There are several reasons why your Mercedes diesel oil became black ALMOST instantly. None of them are "bad" and you do NOT have any problem!

Diesel engines produce LOTS of soot - 10-100X as much as a gasoline engine does, and while MUCH of that soot goes out the exhaust, some remains in the oil and SOME may be recirculated back into the intake (if you car has and EGR system that is functioning). So your car makes more soot than a gasoline car!

Then there is the fact that Mercedes uses an oil cooler in it's engines. That cooler holds about a pint of oil and that oil can't be easily extracted without a chance of damaging the cooler fittings. Then there are the other passages of the engine that hold anothor pint or so of oil. So after you drain ALL of the oil you can (without turning the car upside down and shaking it) there is about a qt of dirty oil in the engine.

It takes about 0.1-0.2% soot (give or take a few tenths of a percent ;-) to make the oil appear black (not just dark, but BLACK), and so if you run the oil until soot reaches maybe 2%, and then drain out 7 qt (leaving one in the engine and then add 7 qts of new to the one qt of old oil left, the concentration of soot in the newly changed oil (once it's all mixed together) will be about 0.25% or high enough to make the oil appear BLACK.

But there is no problem with the oil appearing black - Mercedes counts on that! That's NOT a flaw - it's a feature! That's how they remove the soot from the engine. The oil holds the soot in suspension until it can be removed when the oil is changed. It's just important to remove the soot before it exceeds 2%. Newer API rated CG-4, CH-4 & CI-4 oils are required to hold even higher concentation of soot in suspension (Mercedes re4qires CF rated oils that will usually suspend 2% or a bit more soot), but Mercedes requires that the oil be changed when the soot reaches 2% no matter which oil you use. There are other oil characteritics that shpuld be monitored to establish oil change interval, but soot is the one that will usually limit oil longevity in a Mercedes diesel.

Some manufacturers use filters to extract the soot, and when such filters ARE used, the oil will remain "honey colored" sometimes for 10s or even 100 thousand miles. Problem with that (providing the additive package hold up that long) is that with the kind of oil system and system size that Mercedes uses, the soot filter (the Mercedes filter does NOT filter out soot) would require changing about every 2000-2500 miles and about a qt of oil would need to be added in addition to the filter with each filter change (to replace the oil trapped in the discarded filter).

Most MB diesels that are in good running condition will reach the 2% soot level somewhere between 5 and 20 kmi (depending on your driving style and whether its high speed highway driving or stop and go or short hop city driving and on climatic condition - bad weather USUALLY results in faster soot accumulation). Unless you bother to perform some oil analysis, it's best to stick with the change interval that the factory reommends, but if you DO perform oil ananlysis and don't do a lot of city driving, 10kmi or even longer oil change intervals are frequenty attainable. DO NOT simply assume that you can do that, however. If you want to extend the intervals, perform the analysis and use 2% soot levels to define the change interval. I usually run my mostly city driven cars between 5-7.5 kmi on an oil change and my highway cars to 10+kmi. The former (city cars) often approach or reach 2%, while the highway cars seldom exceed 1.25-1.5% soot.

Marshall