This is from my post on another board. I don't think your system has an overflow reservoir to fill like a lot of older cars. It is self contained. If so, you'll need to wait until it cools then fill directly into the radiator cap....
(my other post):
I looked on the BMW service cd and it lists the approved coolants (other than BMW coolant). There are about 20 but the only ones I vaguely recognize are:
Havoline AFC (BD04)
Castrol ANTI-FREEZE NF
Total Thermofreeze Plus (TOTAL)
GlycoShell (Shell)
Mobil Frostschutz 600
(paraphrasing)
It also stresses that they be nitrite and amino free (for cars since 1986). NOTE, the wording in the TIS indicates that this mostly, if not purely, an environmental move. They also must be neutral to plastic and rubber, and most any metal that might be encountered in the cooling system.
The factory fill is 50/50. They say tap water usually meets specs but if in doubt use distilled. Anything under 40% anti-freeze doesn't provide enough corrosion protection. Anything over 55% does not provide enough heat transfer capability and risks overheating. The boiling point may be higher but the ability to cool through the radiator is decreased. This info is updated as of 2/2004.
So, my take is that you can use anything that is safe for all metals, rubber, plastic and mix at 50/50 with distilled water and you'll be OK. OR just buy the BMW and be done with it.