Yes W123s have oil coolers (pretty sure W126s do also), no, nobody drains them because there is not a plug in them and it is probably not worth the time anyway.
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So no matter how long I would wait, there would be existing oil in the engine anyway. This may be part of the reason for the oil turning black so quickly.
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While this is true the more oil you get out the more clean oil that can get in. Yes this is also the reason why it can turn black so quickly.