All our MBs average 3000 miles a month, and we change our oils once every three months. For the age of our MBs, we use semi-synthetic as I found out that the oil "evaporates" on fully sync. However, if your MB is able to adapt to the Mobil 1 (which happens to be a good oil), then go for it. Do check frequently on the oil consumption if at all you're unsure. Otherwise, semi-synthetic does just as fine.
There is no need to add any oil additives.
Whether you should flush it, I'm incline to say no. Flushing involves a tearing down of some protective layers of the gaskets that may promote leaking, and thus other repairs works coming your way. Its fine to drain the old oil out and add in the new. Do, however, remember to always change your oil filters.
Why is the oil dark brownish? After some running, there would be some form of carbon building up in the engine. Thats normal. Thats why the choice of oil is important for the engine. A good oil will lubricate as well with all the carbon soot than one average one. The viscosity stays intact, and thus protects the engine life and efficiency.
Infact, running an average of more than 15 km non-stop does well in keeping the engine in top-condition than short trips. Long trips will serve to break down the oil and sludge and thus you're have a cleaner engine.
And please, don't add the engine flush and drive unless you really want to strip your gaskets.
126 tailed by a 203, 129 leading the pack.