I'm very curious as to which Redline works best. I'll refer you to a Cobb Tuning article
which actually recommends MT-90 in the transmission on Subarus.
MT-90 is a 75W90 gear oil and is of the proper and recommended viscosity. It's GL-4 and thus is not as slick as the GL-5 75W90 oil.
Before I make my first transmission fluid change, I'd like to collect more opinions.