I used the Wurth Radiator Cleaner with very good results.
Had to flush the system a few times with tap water afterwards. Make sure you set the temp setting to max and lowest fan, so you can clean out the heater core as well.

I also used the block drain hole.

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