I found the following on a list, gtcars.ca ...

It is Subaru's fix for their leaking head gasket problem. They will no
longer pay under the warrantee to replace the head gaskets unless the
additive fails to work. The official Subaru name for this stuff is: Cooling
System Conditioner. Part number SOA635071. The cost is about $1.75. Much
cheaper for Subaru than having a mechanic replace head gaskets.

The stuff is likely the same as Holts Radweld. Container
is the same shape,
same quantity (125 ml), same color (blue) container with a red cap, and even
has the HOLTS name near the bottom. Also says "Manufactured in England for
Subaru of America, Inc."

See: http://www.holtsauto.com/products/index.html
and look for Holts Radweld

I guess someone had to make it :-) It's interesting that Subaru has put some
much faith in this fix.

Best Regards

Dave