I had the same concerns. The simplest test is to open the drain after suction. Last time I tried this, not a drop. Any of the MB approved oils and filters should remove or suspend all the gunk in the oil. If there is so much gunk in the system that the oil can't hold it then something is wrong and needs to be fixed.

I've not tried this on my 230SL, mostly because the dipstick tube is really narrow. It is by no means a modern engine and probably does have gunk on the bottom. But I don't think that the flow of oil out the drain plug will really drag much of the gunk out with it. The only real cure for the gunk is some high speed travel with a fully warmed up engine, then quickly drain (or suction).

Admittedly I'm a convert :-)

...Alan