I have I car coming in tomorrow(05 forester turbo manual) that stalls intermittently when coming to a stop. It a has a CEL that shows a #4 cylinder misfire. I plan to address that first but I doubt that is causing the stalling. In my experience the stalling issue + no related CEL = failing mass air flow meter.

A dead misfire could cause stalling as the engine idles down. Gotta wonder if cylinder 4 is the only misfire. Any idea how many miles are on the car?

just over 100,000 miles. This car has had its fair share of recent repairs: new turbo and new AC compressor are the two biggies. Subaru must have changed who made their AC compressors around 04-05. I have replaced only 1 or 2 compressors (in the past 10 years) on the early body styles and Ive replaced 5 locked up compressors on 2004-06 foresters and imprezas.

Turbo went bad because of infrequent oil changes? I see a picture of an engine full of sludge and an intake full of carbon deposits. :/

Would you suggest an inspection the throttle body/clean. Im not sure what octane of gas they were running.... what are the damages that can incur from running to low of octane in these engines?

On a turbo, extreme amounts of detonation/pinging. The ECU would be working overtime pulling back timing and boost to keep the engine from destroying itself.

Pinging leads to cracked ring lands, cracked spark plugs... most extreme would be the eventual hole through the piston because of pre-ignition, or rod through the block after the piston shatters from very heavy detonation.

 

I would check out the throttle body and clean the plate and bore if needed. I'd also check out the idle control solenoid.