Try redline shockproof and see if that fixes it - its a strange blue fluid so thats where the name smurfs blood comes from. You'll see when you see the container
If it really is the synchro's that are gone, the boxes will have to come out and be stripped etc then worn synchro's replaced. While its all out and open, its a good time to upgrade to a new gearset if more power is on the cards at a later date (or some hard driving, fast shifting etc).
A new box is about 3k or so. From memory getting synchro's replaced is around the $1500 mark depending on whats broken etc (labour to pull the box out, strip, inspect, replace worn parts, assemble, refit to car) but may be more. I know someone in Sydney paid about 2k for that same service but the parts replaced inside make a difference to price obviously.
FYI mines in great condition, doesnt crunch at all, although i am careful on changes, its got a short shifter that i know is also shortning the gearbox life. When (not if) it's destroyed, im going the PAR straight cut 5 speed, comes with everything new that matters, and will cost about 4500. Of course thats for big power, so for anything up to 400kw atw as long as you dont launch it all the time and slam gear to gear, just get a 2nd hand box that you can test first and sell the old one for cheap knowing the buyer will have to fork out a fair amount of coin t get it going again