One other trick I have done, is pour some gasoline in a garden sprayer, then blast it into the upper hole for a while and let it run out from the bottom. Let it sit and drain for a while afterwards and everything will dry out , because gasoline evaporates pretty fast.
I would only go to this extreme though, if the oil had water intrusion and messed up the oil badly. Otherwise, no need for flushing. Just drain it out and let it sit overnight to drain completely.