I flushed my system and used "cheap" prestone. less then 2 months later I had to change my thermostat cause it went bad. Coiensidence, I don't know.
Normal coolant (green stuff, store bought) doesn't contain as much silica or none at all, as BMW stuff does. I don't know if that makes a huge difference, however it may be a safer bet to buy BMW.
You can get a discount if you are a BMWCCA member though.