How old is it? If it's less than 3 yrs old I would just fill it up (any antifreeze will do). Prestone 50/50 is just more convenient because it's already mixed. There is nothing special about BMW coolant. If it is low you probably have a bigger problem somewhere. Antifreeze should remain at the same level unless there is a leak. If the cooling system is already full it's likely a faulty coolant temperature sensor. Which should be easy to replace yourself.