I thought I'd share my experience with oils in case anyone is curious, feel free to comment with your own experiences.
This is on a very low km gearbox (between 25 and 35,000), and all oils are the recommended 75w90. Box was filled with exactly 2L from shifter.
Nissan stock oil: No real dramas, a little finicky to change when cold, didn't have anything to compare it to.
Motul Gear 300: Felt a little smoother all round, still a little finicky to change when cold, hard to tell if that got better or worse. Silky smooth when warm.
Redline MT-90: Noticeable improvement in cold shifting, box feels fairly smooth straight out of the driveway when cold. Perhaps not quite as smooth as the Gear 300 when warm/hot was my first opinion, but after a few drives it seemed to have improved to about the same level.
For what it's worth, the theory behind the change was that MT-90 is the recommended GL-4 oil, compared to the Gear 300 which is a GL-5. There aren't really concerns with modern GL-5 oils eating yellow metals (i.e. brass synchros), but there is the concern that they may be too slippery and may not have the most ideal coefficient of friction for the synchros to spool. I wasn't sure this would translate to anything in practice, but cold shifting has appeared to have improved - which makes me suspect that there might be something to it. I'm tempted to try the Amsoil 75w90 gear oil at some stage, just for the sake of trying new things, but for now the MT-90 seems good.