When it comes to gear oil the big difference between Subaru transaxles and air cooled VW transaxles (and even Porsche transaxles up at least through 915) is the composition of the synchro rings. (drop to the bottom of the post right now for the cheap solution and to avoid annoying tech talk) Most (not all) GL5 gear oils have extreme pressure (EP) additives that will attack the " yellow metal" synchros in the old German transaxles and wear them prematurely. From what I've researched they could reduce synchro life by as much as 50%. This doesn't depend on the gear oil being a dyno oil base or a synthetic base as it's part of the additive package. The thing that most does this is M0S2 or other sulfer based EP additives when they degrade with use/time.
To measure this the test we want to see is ASTM D-130 which tests reactivity/corrosion on a polished copper strip. A gear oil does not have to pass this test in order to get a GL5 rating (it does to get a GL4), but if we can get a spec sheet we can see if it run through ASTM D-130. We are looking for a rating of 1a on that test.
That was why I mentioned the Swepco 202 before as it passes the ASTM D-130 with flying colors. It is a true multi grade 75w90, not a 90w with a pour additive, so it will make it shift easier during warm up and in cold weather. It and gets raves for shifting performance from some folks on Porsche forums. I even stumbled across raves on International Harvester truck forums. The 201 version is similar. Should be able to leave it in there for a long time. Price is around $85 a gallon
The cheap option is Napa Performance Gear Oil 80w-85-90, GL3, GL4, GL5, Part Number #75-210 price is under $8.00 a quart. Slippery link