^^AMSOIL FTW!!! By the way, as a broken record, this topic HAS been covered many times. Search and open 20 tabs covering the same topic and decide on the best explanation you find. That's what I did. And I won't be like one of those assholes who just says "SEARCH!" and leaves. I recommend Amsoil Manual Transmission & Transaxle Gear Lube 75W-90:
AMSOIL Manual Transmission & Transaxle Gear Lube 75W-90
The reason people say not to use GL-5 in our KA24E trannies is because GL-5 has certain chemicals that'll eat up the "yellow metals" in our trannies. Look up "yellow metals." GL-4, however, does NOT have that stuff (or maybe has stuff that prevents that from happening?) and is safe to use. Also, I may point out that in the 240SX S13 FSM, it SAYS to use GL-4. Go download the FSM pdf and save it to your computer. Mine is on my desktop.
If you have any more questions about AMSOIL, feel free to PM me.