Personally I use Bosch or more often aftermarket parts. Generally I use NGK plugs. For leads I go to the local auto store and just get an equivilent part.

I've played with "performance enhancing" leads, etc and noticed no difference what so ever. I think the best solution is to use the recommended parts and keep the service intervals up.

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1985 190E - Gone
1989 230E - Gone
1995 C200 - Gone
1993 280E 120000KM - Built like a brick, the best one yet