Gear oil meant for differentials is typically ill-suited to a manual transmission.
Subarus combine the manual transmission with the front diff (though the automatic has a separate diff) and have special needs for their gear oil.
I know it's almost impossible to find a GL4 75w90 that's suitable for manual transmissions on the shelf of a US store, and the only one I can think of is Liqui-Moly #20012.
If you can get it, look for Valvoline #508216. It's marketed under their professional series (VPS) and may not be readily available to the public or could've even been discontinued entirely.
You could use Motorcraft XT-M5-QS (made in Germany by Castrol), but it's so expensive it's actually cheaper to buy Castrol Syntrans Multivehicle (now called
Castrol Transmax Manual) from Europe, even with international shipping.
Some people have successfully used off-the-shelf Pennzoil and Valvoline Synchromesh on their Toyota manual transmissions, but it is half the viscosity of the correct stuff
I got a VOA done of UltraLub 75w90 GL4 (now called Ultra1plus), and the results were so weak!