[First opel 1981]

Its not the synchros. If it was, then the car wouldn't want to go into gear.
I am almost certain that you can just adjust the clutch and fix it (if you mean it is slipping).

You can inspect the clutch, though its difficult, by removing the boot off of the clutch fork (on the left side of the tranny) and using an inspection mirror to look into the little space between the pressure plate and the flywheel. As I said, its difficult. You would also need to know how much clutch disk you are supposed to have.