Mine was using 1l of oil/200-300Km before tackling the valve stem seals. Exactly the same symptom as yours. The cloud of shame was visible once the engine was warm, and after a short idle at each stop light. Also absolutely no visible oil leak. But once stuff was taken off, well....every seal & gasket was shot. With the beauty covers on and everything else in place, you would swear I make things up. Take a read.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.