Not normal for a properly operating M103, and 15W-40 is, under normal circumstances, not too thick in many climates. My M103 has used 20W-50 or 15W-50 its entire 22-year life, with no valvletrain noise or other adverse consequences.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't