The past couple times I checked the coolant on my 2003 Forester with 239k miles, I had to add 4- 5 ounces (after never having to top it off previously). No signs of overheating but I decided to see if there are any air bubbles coming up in the radiator and found there are a few when revving the engine (see video at link below).

I know that the EJ251 engines used form 1999- 20054 are mostly known for external coolant leaks (this one was no exception- the previous owner had a Subaru dealership replace the headgaskets at 180K miles after the typical external coolant leak from the driver's side headgasket). I think there are a few that ended up leaking internally (similar to the earlier EJ25D's), so I'm wondering if tihs is happening to mine.

The engine is pretty well worn as it uses some oil and has a light knock on acceleration sometimes, so I've been thinking having a lower mileaged EJ251 or maybe a JDM EJ20 swapped in down the road, but I'm wondering if down the road is now.

 

Generally, those have external oil leaks.

 

I have seen internal failures, usually aftermarket gaskets, insufficient cleaning, and/or improper torque sequence on a previous repair is blamed.

Internal EJ251 leaks are usually on previously replaced gaskets. 

Good eye - Bubbles probably means HG is in your future. 

I saw one go 50k on internal leak that didnt progress much so monitor how much it’s loosing and you may Have more time than you thought?!?

Although Im unsure how that happens - seems the combustion chamber has to progress over time and any other leak should have other symptoms that engine didn’t have.