Does it grind or just refuse to go in at all?
It could be a couple of things.
Synchros can be fussy about what kind of oil you feed them because they require metal to metal contact to function. If you are running a synthetic oil or an additive such as Lucas, it could prevent this contact from happening. Sometimes oil can be "too" good.
The other thing is the clutch adjustment. You can have linkage free play that is in spec but still not get enough travel to disengage the clutch completely so it drags. IIRC the air gap between the disc and flywheel is supposed to be .060". If that is the wrong spec someone will correct me.
If it was mine, I would adjust the linkage up until I had little to zero free play and see if it will shift into gear properly. If it does then it was the clutch adjustment. I would then run the minimum spec free play that will keep the release bearing off the fingers but still disengage clean.
If that don't cure it, I would change the oil to a 75w90 that has the synchro friction modifier in it. It usually says on the label if it is synchro compatible. Some guys run ATF as did the factory in some applications but some trans don't like it. (they leak). I wouldn't use any additives.
Kevin
