Pulled a plug today and they are the Bosch F8DC4, so I guess that eliminates those as the problem. Strange. At the same time, I replaced caps, rotors and plugs in the SL 30,000 miles ago, and pulled them off for a look, and they showed very little corrosion.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K