In your red Owners Manual, DeTomaso and ZF say to drain & replace the lube at 3000 mile intervals. Many Panteras have gone their whole 50+ existence on the same factory lube! And almost none have ever had the second shift detent re-greased! But removing it works best.
Marlin mentioned it but the Pantera gear-change system actually has TWO separate shift detents. One is buried inside the ZF and the other is part of the shift-stick assembly in the cabin. Having two detents often causes shift problems. Total removal of the cabin detent frees up shifting on any Pantera, and for a change, the job is easy. D. Quella used to do it free on all the cars he worked on.
Take off the chromed gate and the trim piece (4 metric screws). This exposes the top of the shift mechanism. The shift stick is in a die-cast aluminum housing. Behind the shift stick is a 1" OD snap-ring covering a hole in the housing. Remove the snap ring; inside is a spring on top of a vertical sliding piston. Tease the upside down piston out with a strong magnet, a finger or other tool. It will be caked with ancient dried grease. On the bottom of the piston is a small protruding tip: that is the detent which engages the shift rod going back to the ZF. Bag the piston & spring and put on a shelf somewhere for the next owner.
Close up the hole left when the piston was removed (I used a wine-bottle cork-Chianti, of course!) You're done- replace the trim and shift gate and try your new smooth shifting Pantera- all done while sitting comfortably in the drivers seat and without draining & replacing the brand of ZF lube. But the old lube in your ZF still needs replacing at least yearly!
Note- the above fix and the tach repair in another post today on this Site, were both subjects of our Pantera in illustrated print articles to be found in long-past POCA Newsletters. There are now over 600 such tech articles archived.