Redline makes the proper fluids for your WRX. https://www.redlineoil.com/75w90-ns-gl-5-gear-oil
i owned an 07' WRX I bought new for years until my wife got an Audi A4. It was a 6 speed manual and I did a stage 2 Perrin tune at 60K. I sold the WRX at 136K.
NASIOC use to have a herd mentality on fluids back in the day. There was a member who mixed a bunch of different gear oil he had laying around and created
"Uncle Scotty's cocktail", which I thought was stupid.
I used Valvoline Synpower Synthetic 75W90 in the rear diff from it's first gear oil change. In the 6 speed transaxle, I used Supertech Semi-Synthetic 75W90 gear oil.
That particular Supertech fluid was a but of an oddball gear oil stating not for use in limited slip diffs. As such, that Supertech gear oil provided excellent shifting characteristics
because many gear oils are too slippery and make for bad synchro performance.
I recommend Redline 75W90NS GL-5 for your transaxle. Make sure you put in the NS versionhttps://www.redlineoil.com/75w90-ns-gl-5-gear-oil