Bosch started suppling the W7DC (non-resistor) spark plugs again. Most likely you will have to get lucky to find them or order them from the dealer. There is a MB part number assigned to them, anybody?
Using resistor plugs will work but you add 5,000 ohm resistance to each spark plug circuit. The rotor is 5,000 ohms, the spark plug wire end at the distributor is 1,000 ohms, the spark plug wire end at the spark plug is 1,000 or 5,000 ohm and now with the resistor plugs you add another 5,000 ohms. Early factory non-transistorized ignitions spec maximum resistance in each spark plug circuit to 13,000 ohms. So with everything new you are already near or over the factory limit and have no cushion for age and wear of parts and weather conditions. At the very least you have lost all of your safety margin. Now if you have carbon spark plug wires or carbon coil wire you add a lot more resistance on top of that. Later W113 cars with factory electronic ignitions had a higher voltage and were allowed 19,000 ohm resistance per spark plug circuit. Most of these cars can use the 5,000 ohm spark plug wire ends. If you have to use resistor spark plugs you should upgrade your ignition coil and ballast resistor to supply more voltage. In the end, these engines were designed to use non-resistor spark plugs with the original ignition systems. The next time you have a cold start on a damp cold morning you'll be glad you had a hotter spark from less resistance or a hotter ignition.
