I have red / been told that 'platinum' plugs are not reccommended for-
1 older cars with standard ignition systems which may not produce enough voltage to fire them reliably.
2 some newer cars because the tiny platinum electrodes may cause misfiring at idle.

Having said that, I've been happy using Bosch Platinums (not sure about the '+4' designation) in my 1972 250 sedan for the past 10 years and Nippondenso Platinums in my 1987 Isuzu pickup for the past 7 years. Both vehicles have some type of electronic ignition with
a hotter ign coil than might be in a straight 'point' ignition system. If you can get them in the correct heat range and thread size it probably wouldn't hurt to try Platinums in your 190SL.
Good Luck, Mark DiSilvesto

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DrDKW