There's a seal on the striker rod that dries up or wears, apparently. Very difficult to get to. If you overfill you probably get more seepage there. Not a huge deal except for the annoyance.
Part #22. You'd have to tear the transmission down to fix it.
http://www.carpartsmanual.com/datsun/Z-1969-1978/power-train/transmission-control/5-speed/22