lol @ the people who go to the dealer and buy coolant to top off.
waste. you can just top off with distilled water. but no... it has to be "coolant" you top off with.
fun fact for those who aren't too familiar with how cars work: you can run 100% distilled water in your cooling system.
However for colder climates, coolant lowers the freezing point and provides anti-corrosion properties for the aluminum it flows through.
But just to let you know, you aren't damaging your cars by topping off with distilled water.
As far as the coolant thing goes, my E46 and past E36 M have needed to be topped off every few months with about a cup of water. No leaks, no signs of mixing (also did Blackstone tests) nothing. These are with cars with fully rebuilt cooling systems too.