What KV40 specs have you seen? I'm grappling with gear oil decision myself, and think there is a wide range of specs within the 75w90 offerings. For instance, Motul 300 has KV40 of 72.6, whereas Delvac has 120, and I'm not sure this is the highest (Amsoil Longlife has 129). I found a Chevron 75w85 with 72, so that KV40 is v. close to the Motul 300, while the KV100 is much lower (as you might expect). Meanwhile Motylgear (semi-syn?) 75w85 has 82 KV40. Also, compare Amsoil SeverGear to their LongLife, and you'll see overlap/inconsistency on the KV40/KV100 specs.
By the way, some of this depends on which Subaru, which transmission, which transaxle. You may need to be careful if you have certain manual transmission. There's the whole friction modifier/no friction modifier discussion. 2007 Subaru with Torsen rear specs 75w90, without friction modifiers. I didn't know how "thin" the Motul 300 was at 40c when I put it in my rear diff, but seems reasonably OK. It's possible a higher KV40 would work better. Motul 300 has a very "extreme" VII...so maybe it's a good 75w85 surrogate.