I've used chevron premix and it's worked just fine.

 

Topping off with distilled water is fine (no worries of freezing). Next time you top off, use coolant. Alternating between the two helps ensure you stay close to a 50/50 mix. If your car has BMW coolant in it now, keep using BMW coolant. At a later date, if you drain the coolant, you can use almost any brand coolant as MOST (if not all) modern coolants are phosphate free and aluminum safe. It is also ok to mix coolant brands and colors...they are all compatible with one another.

BMW coolant does not have a magic formula, it is just blue.

 

+ 1 Even use distilled water in the windshield washer bottle - the chemicals in city or well water supplies can be problematic.