Decided to just go with the Subaru OEM stuff. They call it "Subaru High Performance Gear Oil" (HPGO). Picked up at the dealer for about $10.5/qt. Asked if they could do any better on the price, and the counterman said "I'm already giving you a discount off the price we charge service dept. customers. For them, it's $23/qt." Wow. If you're going to have a dealer do ANY work for you, be sure you get an itemized list BEFOREHAND of the prices for everything, and shop around. That's where the term "stealership" comes from, I guess.
Got 5 quarts so I can do the gearbox/front diff, and the rear diff also. Made by Idemitsu - part #SOA427V1700. Doesn't say "GF-5" anywhere on the bottle, but the counter guy at the dealer assured me that it's what they use, and that there is not another MTF/gear oil offered by Subaru. Nothing on the bottle says "synthetic". So I'm assuming it's a conventional product.
Apparently this has replaced the Subaru "Extra S". Found a thread from a couple of years ago:
https://bobistheoilguy.com/forums/ubbthreads.php/topics/3542880/Re:_Subaru_High_Performance_Ge
On paper, it looks to have an inferior addpack to the old Extra S.
Opened up the drain bolts last night after taking the WRX on a 20-min. highway drive to warm the fluid. Letting it drain until I get off work today.
We'll see if shifting improves. It was really just the 1-2 and 2-3 shifts at high RPM that were a bit notchy. Will report back...