The 420 SEL does not have variable valve adjust, so they valves do not adjust to the plugs. I use W8dc, Mike is dead right the w9rdc has a resistor which will screw up your system as the wire connector has a resistor built in. I am sure the 420 will work on platniums but I personally noticed no difference on mine except the cost $32 vs $8. I would buy a set of w8dc's and make sure you gap them to spec.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)