You have valve seals leaking too much.
It is not a big job...most shops will run $400.00.

Only use OE from Mercedes. Some people like the white Teflon seals others say they cause guide wear.

It is not uncommon for the V8 at this age.
Nothing you can do to confirm guide seals but with that volume of oil disappearing and all compressions good as they are, this would be the next step.

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