Food for thought.
Mercedes engineers spent much time making their cars run good, smoooooth, and with good mileage under all conditions.
(this also applies to other manufactures)
If they put a plug in at the factory, they know it will work. I'm not smart enough to second guess them so I will allways use what they use. That way, I will always win.
In my shop (we will work on any brand car) we always use what the manufacturer used. We never have a plug issue. In fact, I won't even work on a drivability problem unless the car has the correct plug.
In a weaker moment, I did try Bosch Plat plugs in my 84 500 SEL (they were given to me by a Bosch Rep). We went for a ride. Came back and took out the Bosch Platium, reinstalled the W9DC used plugs. Took another ride with the rep. Came back and the rep paid me for an hours labor, gave me a new set of W9DC plugs and left.
I knew the Bosch Platiums wouldn't work because I had tried them. The rep said something else must have been wrong, or I had gotten a bad plug and he would pay me to try them again. How could I refuse.
By the way, my 500 SEL just averaged over 20 mpg on a trip from NC to Flordia with speeds usually between 75 and 85 with the Air on and 4 people (all grown-up). I think that is OK for a car this big going that fast for 12 hours and it has W9DC plugs in it and it has 257,000 miles.
Moral to the story.
Always use what the manufacturer used and you can't go wrong.
84 500 SEL
84 500 SEL (307,xxx miles)