What Z did you drive, a 350? It's typical for cold transmissions to be a bit notchy, it should smooth out after it's warmed up. If not, that means that transmission might have been built with the tolerances a touch too tight. Changing to a lighter fluid can help the syncros react quicker, making shifting less notchy, but below a certain point you run the risk of excessive wear. I don't know what weight the trans on the 350s use from the factory, but lets for example if it was 80W90, than you could start off with 70W80 and it should help.
See what Nissan recommends, sometimes if you go too low on fluid weight they may void the warranty. Could just have been the car you were driving, maybe you should try a different one.
If it was an older Z, that seems to be pretty typical.