I agree on it being too low for 20W-50.

Mine does that now with 5W-40. It would bottom out at .3 with 15W-40. For a day or two last summer, when I first noticed the issue and added 20W-50, it never dropped below 1.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black