I've tried Lucas ATF additive, Lubegard red.. and it didn't work.Cheap and possibly temporary solution : Flush the gearbox and change the ATF filter and use Lubeguard additives.
Probably permanent solution : Rebuild the GB
Finally I tried TransX before I rebuild the gearbox..it worked.
My problem was that the car wasn't engaging reverse gear, and when driving forward..it won't shift up.. a bottle of TransX, last year..and now..I just change ATF as usual and the car is fine.
For a RM40 bottle of liquid.. versus RM1.5k to rebuild.. not bad trying.