Thunderguts I like using Redline's ATF-D4 in my manual transmission. You can also use Redline's MTL as well, in the manual BMW boxes.
BG Products make something called: SyncroShift... it can also work wonders on cars with slightly worn out syncros. You might give it a try. I would run your old gearbox fluid thru a scrainer or coffee filter... look for bits of brass(metal) in the fluid - if your syncros are bad.
https://www.bgprod.com/catalog/drive...ncro-shift-ii/