What about a DIY coolant flush? There are generally three ways to perform this:
a. After the system is drained, many fill up the radiator with regular or distilled water alone. Then heat cycle, cool, and drain.
b. After the system is drained, fill up the radiator with a “radiator flush” product and water. Then heat cycle, cool, and drain, and then run clear water through to wash out any traces of the product.
c. After the system is drained, fill up the radiator with a 50/50 ratio of white distilled vinegar and water. Then heat cycle, cool, drain, and then run clear water through to wash out the vinegar. The vinegar is acidic enough to remove any scale or impurities, but not harsh enough to harm your seals and gaskets. Many users believe this type of flush to be gentler than commercially obtainable “radiator flush” products.

Flushes as described above should only be performed by advanced users.

Should I perform a coolant flush? In TSB, 01-166-08, says “Subaru of America, Inc. is proud to announce the approval of the Wynn’s PowerFlush III coolant exchange machine. The PowerFlush III was chosen for it’s three separate tanks that allow the ability to perform a clean water flush of the cooling system in Subaru vehicles. A clean water flush is possible by filling one tank with fresh water, one tank with new coolant and one tank used for removed liquids.” Other machine can/do re-use fluids and as discussed above mixing fluids is a bad thing. So….if you want a machine flush, ensure the shop has a Wynn PowerFlush III, otherwise just say no.

What about topping off my system? Don’t sweat it. Just add water and be done with it as the minor amount of water you add isn’t enough to dilute your overall coolant capacity enough to change much. If you are super worried anyway, you could top it off with pre-mixed coolant, but that’s a little on the “it puts the lotion in the basket” side. And if you think you need to add .1 ounces of Subaru Cooling System Conditioner when you top off…you might want to get a new tin foil hat.