Agree.


Drain, flush out as much as you can...running hose, whatever... Then a flush with DI water...then a final fill with the correct (BMW) coolant + water


(Im on a well and the water is really bad this year (3 years of drought). Even after the softener...so I wont use it for coolant. I have an RO system, I use that. Or bottled DI water. If your tap water is OK, prolly ok to skip a DI flush)