Is it an oil burner? If not, stay with the 0W-20. I have used M1 AFE, M1 HM, PP, PUP, Castrol Edge, Royal Purple HP, and Idemitsu. The only one I can't recommend is Royal Purple HP. It went through my engine faster than any of the others. I couldn't put a specific reason on why, but my personal favorite is M1.
If it uses oil, I'd ignore the Owner's Manual and use 0W-30. A member on the SubaruOutback forum reported that Subaru replied to his inquiry about oil consumption that didn't qualify for the short block replacement, but was still excessive, in his opinion. Subaru recommended using 0W-30. But if your father's Legacy isn't burning oil, I'd stay with 0W-20.