"What about the 6 cylinder engines?"

Although likely not what you are specifically talking about, I just had a
Subaru H6 3.0
engine put in my van. Link here to pics. I'm using regular gas. The ECU
adjusts
engine timing/knock activation so any gas grade can be used. Great
performance
and mileage between 16-20.6 so far, depending on speed traveled. 18.5
average
for the first 3500 miles.

Bob Stevens
'87 Syncro Westy 2003 Outback H6 3.0
http://www.flickr.com/photos/syncronaut/sets/72157623834741032/