Loose valve guides/bad stem seals will not always show up during a compression or leak-down test. There is also a vent for oil vapors which is located between the numbers one and two cylinders.

Do you have much "blow-by" when the engine is running and the oil fill cap is removed? Do you have the auxiliary vacuum pump on your engine?

http://www.peachparts.com/shopforum/vintage-mercedes/254274-why-im-replacing-head-my-m110.html

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