I would stick with the OEM Bosch plugs in the M104 engine. In my 97, the plug is the F8DC4. I change them every 15k miles just because I like to have a look at them and at 15k miles they look near brand new. I am sure they would still be serviceable at the recommended 30k mile service interval.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record