BMW recommends that the coolant be changed every 2 years. BMW recommends that an ethylene glycol based antifreeze coolant be used. Stay with a name brand that is compatible with aluminum and is nitrate and amino free. The coolant should be a 50/50 mix with distilled water to achieve protection to minus 35°F. (minus 31°C.). The concentration should not exceed 60% as this will actually reduce the effectiveness of the fluid as a heat transfer medium and could cause deterioration of coolant passage seals. Using distilled water instead of tap water will also help in the prevention of cooling system deposits.
BMW vehicles are equipped with translucent coolant expansion tanks. There will be marks indicating "MAX" and/or "COLD" levels. There is no need to remove the fill cap from the expansion tank to check the fluid level. Do not remove the fill cap unless coolant needs to be added to the system.