The naturally aspirated flat-four behind a generation of Outback and Forester models — genuinely characterful boxer engineering, paired with a head gasket design that became one of the most widely discussed issues in Subaru's history.
The head gasket material used on this engine proved prone to an external coolant leak, typically appearing as a seep from the lower edge of the cylinder head rather than internal mixing with oil — significant enough that it became one of the most discussed, well-documented issues in Subaru's history, and Subaru ultimately extended warranty coverage for the concern. A revised gasket design addressed the issue on later production and on any example that's had the preventative repair done.
Not a guarantee, but a pattern seen consistently across owner forums and independent-garage reporting.
First signs of head gasket seep typically appear within this range on unaddressed examples.
Risk of more serious coolant loss and overheating rises meaningfully on examples left unaddressed.
A documented, revised-gasket example is a meaningfully different, much safer proposition at any mileage.
This is the defining check on this engine: ask whether the head gasket has been proactively replaced with the revised design, and get documentation if so.
Check for a sweet coolant smell or visible seep near the lower edge of the cylinder head, the hallmark symptom.
Check coolant level consistency over a few days if possible, since a slow external leak may not be immediately obvious on a single inspection.
Model-specific pages, with notes on how it behaves in that particular car.