Engine oil has no business in the transmission....I don't care what brand it is. Amsoil, SYN3, and even Mobile 1.....no business in there.
Gear Lube is the only thing that should be in there. You are looking for Spectro.....I do not know where to find it, but you'll be better off with it than any engine oil.
I am with you there, I have been using amsoil 75 110 in my 6 speed for over 65,000 kms and the 5th gear whine went away right away and the shifting has never been smoother