I don't think you have to drain some of the tranny oil. if you have over filled it maybe. but you say that the tranny have shifted this way for a long time. if the problem came after an oil refill, maybe thats the problem, but not likely. the level will drop when engine is running. this is because the oil pump is building up the pressure that the tranny needs to engage the clutch inside the tranny. and the other way when you switch of the engine, the level will rise.

Mercedes560SEL
NORWAY