On my 560SEL I routinely changed the plugs every couple of weeks, for about 2 years. This was due to worn valve stem seals (yes, they were later replaced). I became very good at changing the plugs and could do all 8 in about 10 minutes or less; I'd clean the plugs up and reuse them 2 weeks later. I don't recall any difficulty at all with this task.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280