Bentley recommends replacing the O-ring on the drain plug, which is not listed a separate part. I just replaced the entire plug, they are just a few dollars. Torque to 22in-lb.
Also, BMW warns not to subject the pump to straight coolant, it must be mixed with water outside the engine.
For service in Arizona, I went to 60/40 water/coolant with 1 bottle Redline water wetter.