I wouldn't hold your breath, James. MB's not going to posture themselves on an issue like this.

Bill is right, but one must temper his findings. Note he carefully said “on some configurations”. On some cars a resistor in a high voltage circuit can increase impedance AND capacitance, often creating a standing wave. HAMs know it as SWR or standing wave ratio. Again on some designs it can reflect back into the electronics and cause all sorts of mayhem. On some designs it is dealt with fine by run-time self adjustments. Some motors have local coils, one for each plug and no run length of plug wires at all. Some have seemingly mile-long plug wires into aged rotors, some into electronic controllers.

Bottom line is MBs stance will be to recommend whatever plug is in parts inventory at the time. I've seen "R" plugs come and go as OEM recommended and supplied. Worse, it varies by type. You never know what you’ll get, therefore you’ll never know what’s recommended. How many of you remember the platinum craze?

If you ever think you've put your finger on the issue, the plugs handed to you by MB parts will then change and the whole thing starts again.

I still agree with Bill that R plugs on recent MBs is pointless and may even cause problems. But you’ll have a hard time confirming it across the board for all cars and all installations. IMHO this issue borderlines the great oil-type debate started by Henry Ford in 1915