You should stick to the recommendations outlined in your vehicle owner’s handbook. This is particularly true with regards to modern vehicles, whose cooling systems require specific types and concentrations of engine coolants. Mixing coolants can alter both of these. When your car is serviced, the technician will inspect your cooling system including the coolant and if necessary they can test to see if it is still within required specifications. If the coolant does not pass the test they will recommend a flush and replacement of the coolant.