I use 87. That is the minimum recommended by Subaru. all Octane means is the amount of resistance to burn of the gasoline. This is why turboed cars or cars with really hight CR need to use higher octane gas. The air-fuel mixture is being compressed more and PV=nrT, which means that when something is compressed, it gets hotter. if the gasoline ignites itself, it can be very damaging on the engine. this is called knock. you won't hear it in a modern car. the knock sensors are sensitive enough that they will pick it up while it is still minimal and retard the timing to prevent it. of course, this means less power. anyway, back to the topic at hand. if you plan on using a higher octane of fuel, reset your ECU or you will probably get a decrease in performance. the Subaru ECU is very adaptable and once it gets set on a certain octane, switching to something else will fuck with its head. FWIW, I get shit for mileage in the city, but it does well on the highway. i also get better mileage on pure gasoline then the 10% methonal crap the EPA forces on us in most major metropolitan areas.
