Reset service light indicator, engine oil life Mercedes Benz M – Class, year: 1998-2004, oil reset instructions:

Switch ignition on and immediately press knob to left of speedometer twice within 1 second.
Present status for days or distance is now displayed. Within 10 seconds, turn key in steering wheel lock to position 0.
Press and hold button to the left of speedometer, while turning key in steering lock to position 2 again. The present status for days or distance is displayed again.
Continue to hold button until you hear a signal. Release button.
The service interval should now display the reset interval.