Was changing my coolant today in my '99 540, following the Bentley Manual procedure, which said to basically "remove" the radiator plug. My drain plug was blue in color, and didn't look much like the photo in Bentley (prob a '97 or '98 540). It sure as heck didn't unscrew.
Anyways, couldn't turn it by hand, so I got some pliers on it, and managed to turn it 1/4 of a turn, but that was it. Bentley didn't give any other detail, so I went back to give it some more pliers torque and ended up breaking the little ears on the cap, but didn't move it - AArrgh!
After playing with it further, I realize it turned that 1/4 turn easily, so my next theory (correct) is that it's a quick-release plug: 1/4 turn to the left and then you pull it DOWN. It was stuck, but sure enough, I got the pliers on it again and pulled it down and out. Ta-DA! No thanks to Bentley, I fortunately didn't screw up my plug too badly.
BTW, the flow out was only a trickle.
But then Bentley was correct when it said to put a 5 gallon bucket under the engine drain bolt, because when I removed the first of those, it was like a garden hose pouring out coolant!
In summary, most of the time I've gotten "stuck" or screwed up something working on my 540, it was because there was a "trick" to know, or some unseen quick release, that made it actually easier, such as with the radiator drain plug. These cars are really great to work on, you just gotta know all the little tricks, almost like secret/cheat codes in video games...