[j3rf]

You are correct in that it's not likely to be a head gasket issue even though some people immediately default to that because Subaru. Chain motors when they suffer from this usually have issues with the upper oil pan o-rings or CVT cooler. Being a manual, can't the CVT cooler. Unfortunately only way to confirm is pull engine, remove front chain cover, remove lower and upper oil pan and inspect the orange o-rings.

 

[j3rf]

You are correct in that it's not likely to be a head gasket issue even though some people immediately default to that because Subaru. Chain motors when they suffer from this usually have issues with the upper oil pan o-rings or CVT cooler. Being a manual, can't the CVT cooler. Unfortunately only way to confirm is pull engine, remove front chain cover, remove lower and upper oil pan and inspect the orange o-rings.

@CWP18MT Had another thought about this while at work earlier, since we have one in the shop currently. Inspect the rest of your rubber hoses in the cooling system. If you find they are extremely swollen and soft, then it's possible the oil has been mixing in the coolant for a while. If you find that they are normal looking and feeling, you should try cleaning out the reservoir and monitoring if it comes back.