Sorry but too many idiots…
Sorry but too many idiots driving that shouldn't be. This issue with the spare and awd in general is circumference of the wheel. That needs to be uniform across all wheels (also why reg rotation is more important on awd). Subaru is just being cheap and laywereez warning about using spares. Easiest is to just get another OEM wheel and put exact same tire on it as your other 4. DO NOT RELY ON OR USE DONUT SPARES!