According to Amsoil, if you are using their Signature Series synthetic oil and their EaO filter, the drain interval is 25,000 miles or 1 year, whichever comes first (normal service), or 15,000 miles or 1 year, whichever comes first (severe service). They have a pdf file on their website that defines normal and severe service. https://www.amsoil.com/lit/g1490.pdf
If using any oil filter other than the Amsoil EaO, the recommendation is to use the OEM oil change interval recommended by Subaru.
Paying $10. for a trial 6 month membership as a preferred customer knocks the price of the Signature Series oil down from $12.05 to $8.55/qt, and the filter drops from $16.65 to $12.35, so about $55 for an oil change once a year (5 quarts and a filter purchased). Annually the preferred membership is $25, so if you have more than 1 vehicle, or other power equipment you probably come out ahead going the preferred customer route.
I'm not a distributor or connected with Amsoil other than being a customer. I'm just passing along info.