I'm all for going for the best when it comes to some fluids, but only if there's a reason. For instance, I plan to dump the factory fill and add Motul Gear 300 in both my Forester XT and Legacy, then not change the lube again for a LONG time, if ever again.
Dumping expensive (~$20/L) lube often seems pretty foolish in such LIGHT, LIGHT, LIGHT duty applications. Might as well just use ST, if you want to drain it every 30k miles.
Hmm. said vehicle is a new 2019 and you don't want to use the factory approved gear lube. Do you have a legitimate reason to dump the warranty if you have gear problems? Ed
What in the world would make you think the warranty will be dumped???
From the 2019 Impreza owner's manual, the only requirement is that the gear oil have an API classification GL-5 (75W-90)
-----90 is also allowed above freezing (i.e., N/A for the vast majority of Subaru owners)