Very interesting,

First, Mercedes designed the engine to have an optimal oil level- full is actually the absolute max, and min is the absolute min.

I would fill it no more than the midpoint, as MB intended. That will reduce your oil consumption. Leakage- you have wash the engine and find the leak. Some o-rings are possible, V/c gasket, even headgaskets- you'll just have to look.

It's normal for diesels to have some blowby. Drive it hard, feed it good fuel and use diesel rated oil. All will minimize blowby by seating the rings.

Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E