I cannot see (in principal) why platinum plugs would not work on a Mercedes. They were wonderfully foul-resistant on my old oil-burning Renault. But....

The real problem is that the Bosch platinums seem to be only available in a resistor plug, and our 109E (and probably lots of others) needs a non-resistor plug. The resistors are in the plug caps. The Bosch listing ignores that, and lists resistor plugs as fitting our cars. Not so!