When was your vehicle first placed in service? You’re likely already out of B-2-B unless you bought it new and it was left on the lot…
Here’s what your manual has.
View attachment 227625
Note that the diagram for oil viscosity shows that 5w30 and 5w40 (and all 0w30 or 40 by default) are
completely interchangeable at all engine temps. I’ll tell you from experience, and many others on here likely will as well, that using an “API SN Resource Conserving” or an “API SN+ Resource Conserving” oil is a good way on a hard-driven turbo vehicle to have turbo, timing chain, or timing chain guide issues not long after your warranty expires and the dealer says you’re on your own…
You’ve had some experienced folks give you some good advice. I’ll just say that the dealer will be more than happy to take your ~$7k for a new engine without one flinch of their conscience even if all you had ever used was Subaru-branded oil.
Considering you’ve got an EJ25, I’d personally be running a quality ACEA A3/B4 0w40 or 5w40, and sample it at 4k miles. That’s a 20+ year old engine design that’s known to need higher viscosity under duress.
Page 12-5 in the manual shows the oil recommendations:
https://cdn.subarunet.com/stis/doc/ownerManual/MSA5M2105A_STIS-Opt.pdf