Manual Gearbox oil :
GL4 ONLY
75W80
Elf NFP (or equivalent).
The bush kit (at gearbox end) :
Gearbox does not have to be removed. It is purely rod based.
Unlikely the bushes alone prevent gear selection. They may create gearshift play or vagueness, but not a total selection failure.
Note that the selection rod is adjustable for both angle and throw. This could affect gear selection. However, if its unchanged from factory, its unlikely to have changed. If it has moved, or has come loose, there are specific measurements in the "Haynes book of lies" on how to adjust - but i'd say just play with adjustment until it works.
From what I can fathom, you are running ATF in a manual box, so rectify that first!