Yup, Walmart is selling the 5-qt jug of Mobil1 0-20 full synthetic for $26.50. On eBay, it looks like I can get the Subaru-branded filters for approximately $7.50 each, if I buy a 3-pack (and $6.67 if I buy a 6-pack). So it looks to me like I can do my own oil change for under $40, and know that I put in the right quantity of the right product.
It looks like the non-Subaru oil filters sold at Autozone are similar in price to real Subaru filters from eBay, so I'd just stick with the Subaru filters. I don't know who OEMs the filters to Subaru.
Still deciding between a Fumoto valve and top-side oil extraction. Would only go with the valve if I could reach it without jacking the car or driving up on ramps.