Coolant is Critical to your Subaru:
Avoid the Use of Coolant Flushing Machines
Subaru does not recommend the use of any flushing machine or flushing agent under any circumstances. If a flushing machine has been used to service other brand vehicles with copper radiators, a chemical reaction between copper ions and Genuine Subaru Coolant may occur. This could also cause future clogging of the radiator.
If a flushing machine is dedicated to only Subaru vehicles, it is still not recommended as there is no way to know that the coolants being removed and processed through the machine during servicing are or were exclusively Genuine Subaru Long Life Coolant.
If flushing is required, only use fresh tap water. Do not use hard water as it will create calcium build-up, which will clog the radiator.