Advice I received from a Mercedes Tech at a well known Dallas Mercedes dealer.........

Older plugs were made to run hotter, whatever that means. Due to the type of fuel used at the time the plugs would foul quickly so the plugs were built to run hot to burn off the fouling. You still had to change plugs about every eight to ten thousand miles as they were used up by then.

Newer plugs are for engines that use a different formulation of fuel from the 60's. Since the lead is gone the plugs don't foul out as quickly so they can run cooler. And they are made to do just that.

So putting a cooler plug in where a hotter plugs was originally is supposed to make the cooler plugs foul out quicker. At least that's what I was told. I don't know enough about the subject to make any decisions on this.

But in the case of the older Mercedes there is another reason to stick with the older number plugs. I installed newer plugs in my 230 and suddenly I was hearing the tick, tick, tick through my radio on AM. The Tech said to look at the metal shielded plug caps. On Mercedes the resistor is built into the metal cap and nowhere else. And by using plugs not designed for that car I was screwing up the entire ignition system. Or perhaps I should say the radio system.

This info is about 30 years old so any additional info would be welcomed.