Bosch is good, genuine MB is a 5 year cap. Bosch typically is 3-4 years.

New plugs every year
Cap and rotor every 3-4 years
Wires every 6 years

Don't ever adjust the lambda or the eha, that is a bandaid fix, and you will never get your car to run properly. That 300se should get at least 17mpg, perhaps almost 20 with highway driving. This is the most important indicator whether your m103 is running properly.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2