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VERY good advice. I had this problem when replacing the trans on my dad's Z31. Needless to say, that was a little nerve-racking, since I'd already reinstalled the trans before trying to remove the filler plug.

I use the GM Synchromesh fluid in my trans (actually, the GM stuff is made by Pennzoil, and they sell their own fluid at parts stores for less than GM does, so technically I use Pennzoil Synchromesh), which isn't a Z, but most all synchromesh manual trans use the same design, and it also calls for a 75w90 GL-4 gear oil. It works great. I've also used Redline and Amsoil gear oils, all felt about the same.

Be careful when selecting the trans fluid. The spec for the trans is a 75w90 oil meeting GL-4 specifications. MAKE SURE what ever fluid you use says its OK for applications calling for GL-4. Most synthetic 75w90 fluids are GL-5, which may contain sulfur, which will react with the brass synchros and bushings in a really bad way. Redline MTL and MT90 are both safe, but fluids like the Mobil1 75w90 or many other common name 75w90 synthetics are not.