Do not worry about the color of the oil. It is suspended carbon--carbon being a ( maybe THE) by-product of diesel combustion. The carbon is very fine and blows past the rings, and gets picked up by the oil. Thats a good thing--the oil is doing its job. You want to keep all that stuff in suspension so it drains out with the next oil change. Having clean oil, but a sludge-covered mess inside the engine would be bad.

If it REALLY bothers you, here is a suggestion. Drain the oil when the engine is hot. Let it drain as long as you can, over night is good. With the drain plug still out, dump some kerosene, or even diesel into the oil filler. If you watch as it drain, it will be black, too. Maybe you can squirt some of the same stuff up into the crankcase thru the drain holle. Let it all drain out; repeat until it runs clear. Install the filter, and the drain plug. Dump in 8 qts of cheap HD motor oil, and run the eungine for an hour or so. Drain and change the filter. Repeat as necessary.
Your final fill of oil needs to be the proper diesel-rated oil.
It will look remarkably clean .......for a day or two, maybe even a week.

Also, " Welcome to the site"!

As the new owner, I suggest (strongly) that you replace ALL fluids unless you have comprehensive records. The previous owner probably had gotten to the point where he refused to do any maintenance for years.
Also, until you can visibly inspect all of the steel brake lines, STAND of the brake pedal ( engine running) before you leave the driveway/garage. If a line is going to blow out, you want it to happen here, and not out on the highway when you might really need to stop.

I am not sure why locking the doors affects the cruise control. I could understand that in a W123, but not in the W126, S class. The vacuum for your lock system comes from an electric vacuum pump in the trunk, next to the spare tire. I do not believe it is connected to the engine system in any way. Besides, I think your CC is electrically rather than vacuum operated.

Anyway, again, welcome to the Forum. All your questions will be answered.

One more thing, I think WalMart sells the synthetic Rotella T at about $14 or so a gallon; not quart.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags