There is a Subaru additive that is about $2.00 at the dealership. It's in a little blue bottle. Suck some coolant out of the radiator and put this in (don't put it into the plastic expansion tank). I add this every time I do a coolant exchange on my Subie.
I've done 3 head gasket jobs in the past year and I'm going to say its a time consuming but straight forward repair. I picked up my Outback Wagon daily driver with 190,000 miles on the original head gaskets. They just started to go bad.
The gaskets have some sort of graphite coating on them that appears to break down if you don't change the coolant as scheduled. Make sure you use the GREEN Subaru coolant or just have it done at the dealer. Last time I checked it was about $32 per container and you only need one to do the car.
Here's what else I've learned:
The 2.5L '97-'99 DOHC cars leaked internally
Your motor, the 2.5L '99 - '01 SOHC cars (phase II motor) leaked externally. Take a look from under the car and see where the head meets the block. See any green liquid?
I've also done a compression check to find a leaking HG on two of the three motors I've worked on. They should be about 170-175 psi on all 4 cylinders.
Some sites for more information:
www.subaruforester.org and
www.subaruoutback.org
Here's a picture that might help explain things:
Those two lower rectangular areas on the left and right are the oil return passages from the head. The two lower valley's next to that center hole (that hole is where the head bolt goes) is where the leak starts and it runs to the back of the head. That's what happened with the motors I've worked on.
You can see from the cylinder head picture that coolant doesn't exactly flow in that area. It tends to pool more than anything (this is TOTALLY my opinion).
That blue attitive is all that I would do besides looking at the heads to check for leaks during routine oil changes.
