The platinum plugs should be used only in the MB engines designed for them. There have been a number of discussions on this board regarding plats vs copper cores. As a result of one of the earlier threads, I pulled the plats from my 4.5L and replaced them with Bosch copper cores. Idle improved, seemed to run better and a slight increase in mileage.
Best bet, if the engine isn't designed for plats, stick with copper
73 280 SEL 4.5