That chassis originally took F8DC4 , non -resistor plugs...but you can not readily find them anymore .
so..they now give you the FR8DC Resistor replacement plugs...the problem is there are already resistor ends on the plug wire connector ends, so you wind up with double the resistance .... which they claim works fine ... but many are finding they don't work as well b/c you wind up with a colder spark , and that may also effects the caps and rotors.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2EI1B4MYE2EZ1A159R&year=1993&make=MB&model=400-E-001&category=F&part=Spark+Plug

Many guys are switching over to NGK replacement b/c they still make the non- R repalcement with good results.
Just an Opinion....

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A Dalton