Thanks for all the replies. I don't think I'll be covered under the warranty for 3 reasons: 1) I'm not the original owner of the car. I bought it used. 2) The warranty covers 1999-2002. My car is an 2003 but it was built in 12/02 so maybe I could squeak by. 3) I don't have any official record of a dealer installing the coolant additive. My local mechanic put it in.
My local mechanic got a whole lesson on Subaru head gaskets last week. He replaced my radiator thinking that would solve my overheating problem. He spent hours doing all kinds of tests and reached out to many other mechanics.
It has only been three days on the road and the car seems normal again but I am leery driving it and I watch my temp gauge like a hawk. I was told that the leak I have is very small so I'm hoping this conditioner will hold for some time. I only have 2 or 3 payments left and this has to happen! Murphy's Law!