I think Royaiii makes the most important point. Synthetic oils are detergent oils by nature. Therefore, if you have an engine that was not cared for in the past and thus has developed a severe case of "gunk" on the inside, the synthetic oil will clean and release this crap from the internal parts into the flow. This creates an outstanding opportunity for clogging of the filter and/or the oil channels within the engine, which then leads to a really bad F'n dream. I would be hesitant to change to synth in an older car unless I am VERY VERY confident about the cleanliness of the internals and I know the oil change & maintenance history of the car very well.

As for Synth vs. Dino oil. Who really knows? Unless your racing your 900hp machine through the streets of Monte Carlo, I don't think it makes a damn bit of difference.

When you change your oil, there are still pockets of residual oil in the engine. The residual mixes with the new stuff, and makes it darker. The longer you go between changes, the darker it will be post-change. Some engines store more residual than others. Auxillary oil coolers and anti-drainback valves in particular can make for additional residual capacity.
Since this is your first change, you may want to change the oil a second time just to be sure you get off to a fresh start.

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1986 300E 5-Speed 240k mi.

I wouldn't be that much worried about the detergent nature of synthetics. It's a very mild feature that eventually after the repeated adminstration of synthetics make the engine cleaner but it is not going to "Drain-O" your engine. It's not going to dislodge everything in the engine.

Why I don't use all Mobil 1 in my 300E is because it does not have impeccable maintenance as well as a diet of 87 octane gas for 7 years of its life. The burning of the fuel is not 100% and a less accurate air/fuel ratio. The newer 3 valve Benz engines can run 20,000 mile oil changes because they burn fuel cleaner because of their twin-spark design as well as an ignition coil for each cylinder and MAF sensors. Most likely your newly acquired car, does not have this feature. Also my 300E, leaks a lot of oil and I go through a bottle of oil a week. Not only wouldn't it be economical to burn this oil but also I don't have time to contantly by Mobil 6 packs. There's my 2nd 2 cents