A search finds this site, from a Subaru specialist in Seattle, All-Wheel-Drive Auto, with excellent technical discussion of Subie HG problems by Justin Stobb (very readable, except for confusing plurals and possessives):
Apparently there were two generations of head gaskets, with different problems. Most of the subsequent comments concern 2002 and older Subies, but without reading them all I found this query and Justin's answer:
Ian Says: ?November 3rd, 2008 at 4:58 am
Hi, I?m researching 2004-2008 Subaru Outbacks. I?m aware the hp differs in each year and there is a turbo. Power is not as important to me as being mechinally [sic] sound. Have you heard of any problems with any engine in this range of years? Or are they too new to tell yet? Thanks a lot for your awesome site, Ian.
Justin Stobb Says: ?November 4th, 2008 at 5:18 am
Hey Ian,? Thanks for the feedback on our site. ?The engine is basically the same from 2000 to 2004 and from 2005 to 2008. ?The big difference is the variable valve train components in the 06 and newer models. ?I would probably steer towards the 05 to 08 platform, the 2.5l is basically the same with some revisions but I feel that overall the 05 and newer seem to be pretty solid at this point.?We saw the 2000 models with gasket leaks by 2004 so to not really see any issues with the 05 models makes me feel pretty good about the mechanical aspects being improved.? Justin
BTW, Justin identifies battery corrosion as a contributing cause of the HG failure! Also, the second-generation gaskets were helped by a coolant additive Subaru developed, which should be added any time the coolant is flushed and replaced. And there are some inherent difficulties with a flat engine, which doesn't have a lower block for fluids to drain into when the engine is at rest. (However, Justin adds that the boxer engine helps with the AWD and lower center of gravity, so there are trade-offs.) Read the article.
Anyone know of a comparable Subaru expert in the Metrowest area of eastern Massachusetts (between Boston and Worcester)?