I used Beck Arnley Extended Life Blue when I put in a new radiator. I paid $18 a gallon from BAP. You can also get it from O'Rielly. Beck Arnley says its OE spec. I think they may be an OEM. They make four different coolants for Japanese makes. Two are the red and pink for Toyota spec and the other two are the green and blue Subaru spec. They also make US and European spec anti-freeze. I'd give the link with more info, but not sure if it's permitted.
I was skeptical of the Subaru cooling system conditioner, but not any more. My radiator upper tank developed a five inch long crack and was squirting out a stream the size of the wire in a large paper clip. I put in a bottle of the conditioner while topping off my coolant and the stream stopped within a few miles. I still had seepage because the side of the tank flexed under pressure and would break the seal but coolant didn't squirt out any more.
After replacing the radiator I took it apart. I found no sign of sediment, clogging in the core or any build up at all except at the crack. I'm a believer.
BTW, I measured coolant capacity of the block while I was doing several pure water flushes. With the radiator out I clamped the radiator hoses closed and one gallon of water filled the block.
On another note. Beck Arnley recommends against any kind of cooling system flush agent. They say it will contaminate the coolant. And they don't make flush agents.