I'm having similar issues with the old gear oil in my trans, its probably a 200,000km old factory fill

, any good oil for older transmissions?
The syncro rings are slow to work and fast gear shifts result in a uncomfortable click as they are forced into mesh.
For industrial machines I run lighter oils like ATF or Hyspin if I want less resistance and higher speeds, but if syncro's hate low viscosity an ATF fill would not be a good idea ( I was planning on doing what Honda does and run ATF as an experiment in one of my boxes)