1986 N/A randomly pops out of 3rd gear

Need to make sure it's GL-4 oil. GL-5 will eat up your synchros. Most people use Redline MT90 in their 71C transmissions.

Also, read the stickies… http://z31performance.com/showthread…-can-be-a-pain

ok thank you I'll get enough fluid to do my trans and rear end as well. would this possibly solve my popping out of gear?

Fluid won't help. Detents, or spring loaded balls and indention, on the shifting rails and syncro hubs hold it in gear. Since one ball/spring setup handles 3rd and 4th together, on the shift rail, it's unlike that it is that… Oh course, the syncro hub assembly between 3rd and 4th also share the same little spring loaded tabs, i call them dogs, i think the fsm calls them insert springs. So, odd that you are having trouble with one but not the other. But it could just be all sorts of shit worn out, moving around. When things can thrust back and forth in the box and the shift fork doesn't move, this kind of stuff can happen.

Did is start before you checked the oil level. Sure you didn't pull a small plug out of the side of the gear box looking for a filler plug? Maybe a little spring and ball fell out : )

I think this is the correct plug the 3/4 check ball, should have a spring then a ball under the plug.

just saw that they recommend gl-5 80w90 for the rear so I just need the gl-4 for the trans

no I haven't checked the oil at all. I just assumed that would be the easiest thing to try first.