I wanted to share a few things I learned chasing down misfires and idle issues in my 07 2.5i .
Here's what made my car run better than new at 70k miles. Some of these things are interconnected. I'll leave it to you to connect the dots.
Obvious: Cylinder #2 spark plug well, or spare oil stash type place. Yes, this one seems to be the worst since the oil fill is above it. It was like adding oil seemed to half fill #2 and 1/2 go into the crankcase. New valve cover gasket, and plug well seals fixed this. I did all 4 just because the fel pro kit was 30$ and "while in there.." Vacuum leaks fixed too! If oil is coming out... air is able to come in.
Not obvious: Spark plug wires from factory tested perfect...... Car had slight misfires at idle and engine rocked -- not enough to slow the crank speed and increment the misfire counter, but enough to feel and visually notice at idle....
Subaru decided NOT to use dielectric in the coil pack recepticle, this causes a few issues. Moisture can get in. Worse, arcing can(read does) happen, and especially when #2 (the one prone to the oil filling the plug hole most) is oil filled, the spark will often arc between the plug end and coil pack. My coil pack was charred inside, as was the plug end there. cleaned both off well, cleaned all 4 coil pack recepticles with folded up sandpaper and compressed air/alcohol. Major difference.
Spark plug wires tested good.... why do NGK wires make the car idle smoother and run better? Something was bad, likely oxidization/degradation of the spark cable internal from years of heat cycling.
How not sensitive is the subaru misfire counter? Well I ripped one of my old plug wires seperating the end on the plug from the actual wire.... I was able to jam it back in and the car ran ok, but missed hard at times... WITH NO MISFIRE COUNTER INCREMENTS.
Not obvious: Subaru uses a MAF... but slips PCV hoses and other things on with no real host clamps, or clamps you could undo with a pinky and thumb then slide off... will these magically not expand under heat and leak vacuum? Of course not.....
Solution? Add hose clamps *EVERYWHERE*. See a spot with no clamp? Add it.
Oil consumption...... This was my experience. My car ate oil, 1qt every 1000 miles or so of mobil 1 5w30. Even after fixing the leaks. Problem? Oil control rings locked into ringlands with carbon, compression may have tested "ok" but it should not eat THIS much oil, even though they do by design..... Fix? Seafoam and carb cleaner. pull the plugs, spray some in the bores while motor is hot and off, rotate motor a few times... repeat, but don't be dumb and spray enough to hydrolock the cylinders or dilute your oil to the point of spinning a bearing or causing damage. I like to do this one before each oil change. You not only burn the carbon off the piston crowns and valve faces (that soaks up your fuel charge AND can cause hotspots/detonation) but done right with the turn it over and spray more in the plug holes, you get the seafoam spray/carb cleaner worked down into the ringlands to break up the carbon locking your piston rings in place.
Up side? less oil consumption, more power and compression, better fuel economy.
Hope this helps some folks who wonder how/why the car is running so strange, rough or just not up on power like it was.
This stuff worked wonders for me.