Coolant is lubricative so I would not add water alone to the system. It was not designed to pump that fluid. It might not hurt, but not doing it definitely wont hurt.
Define 'wrong coolant' too please. If the car had "Jap spec" coolant in it I would just drain and replace. The main difference will be the color. I think you made a good choice by choosing to return to OEM though.

 

I think they meant just to flush before filling up with the Subaru Super Coolant, not running water permanently? Unless you meant not to run the car at all with only water? Is it pointless then to fill and flush without starting?

@kieferj231 If it’s below freezing where you live then I’d advise against it. Was in a similar situation recently and wanted to do a flush but decided not to due to Chicago weather.

 

Just flush it with the new coolant, no need to use water. Remove old, add new, let it cycle and repeat that about 3-4 times max then just drive the car. Unless the old coolant is going to hurt the engine, you should be fine with the above.

 

As careful as I am, I never flush with coolant, only water. Long ago due to cost, these days not to unnecessarily discard significant pollutants.

I'm not promoting it, but as a kid I ran water in cooling systems many times for whole summers with no identifiable issues. Modern cars run hotter and really need coolant - a lot more than lubricant or anti-electrolysis additives.


Heck, we have members who are more than reluctant to spend money on great oil and 3K OCIs. Here were are worried about a few minute run time with water in the cooing system? Believe me no dealer or shop is going through gallons of coolant to flush your cooling system. OK look hard someone will find one that says they do, charges for it, and then probably doesn't . . .

 

Using distilled water for a coolant flush is a good idea. It helps remove any remaining old coolant and contaminants. Just make sure to follow up with the Subaru Super Coolant for the final fill. It's all about getting a clean system for optimal performance. Best wishes.

 

Water isn't a great cleaner. If you cooling system is dirty enough to need cleaning, the use a flush product. Otherwise flush with water.

Coolant is mostly water, and some of us still buy coolant the only way it used be be available, as a concentrate, which you mix with . . . . . wait for it . . . . water! ! ! !