The English language factory service manuals seem to recommend GL-4 transmission oil regardless of whether it is for Warner type synchros or Servo type synchros. However, I have Japanese factory service manuals and transmission service manuals from Nissan Sports which recommend GL-5 for the Servo type synchros. Seems to me that there's a mis-match there.
Porsche recommended GL-5 for their transmissions using the Porsche/Servo-patent steel synchros, and there's traditionally been a lot of debate in the old Porsche world about what is best. I think the experts go with GL-5, and their reasoning seems to make sense. In my personal experience (for whatever that's worth) a good condition Servo synchro trans (either A or B type) with GL-5 is a joy to use.
The two synchro types are fundamentally different, which is at the root of why they chose to go with the 'softer' action Warner type for the North American market. To cut a long story short, Nissan were not convinced that the USA's mass market customer in particular was sophisticated enough to enjoy the rather mechanical synchro feel of the Servo type. This went hand-in-hand with the softer springing and damping, slower steering ratio and wider ratio gearing chosen for that market.
You might want to confirm that though. Plenty of 'our' cars have had later Warner type 'boxes fitted when their original 'boxes started to need attention. The period go-to (because they were cheaper than a rebuild) was a 'box from a rusty 280ZX...
All I can say is that it it IS a Servo synchro 'box, then a GL-5 oil change is worth a try. Worn-out Servo synchros are just as bad as worn-out Warner synchros, so an oil change is not going to fix that. However, a change to GL-5 might help a partially worn Servo 'box to shift a little better.