You dont *HAVE* to use any certain thing.
You could run good ol hose water in there as long as you dont have any freezing temps.
The issue lies in the metals our particular motor is made of. Ours is aluminium and I think some other metal. Typical coolant might corrode the interior of your engine so eventually it will cause serious problems that would be near impossible to repair.
The cost difference isnt that significant when you think about how long a coolant flush actually lasts, so just go with BMW coolant. IMHO of course...