What Mercedes recommends, if you go by what their electronic parts catalog says, are the following:

1) Bosch H9DCO
2) Beru 14K-9DUO
3) Champion S12YCC

Many MBz enthusiasts also recommend the NGK BP5EFS plugs for the M103 engine. All four of these plug models are copper, non-resistor plugs; that seems to be the key, although there are ample testimonials here and elsewhere that Iridium plugs work well. I just don't see that the high cost of Iridiums makes sense for an engine that is recommended to take copper, non-resistor plugs.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't