Welcome!
Many of our old beloved 915 boxes have pretty beat up first and second synchros by now. The 915 in general takes a bit of tenderness and smoothness to get along with. If you can live with it I would wait for a clutch work.
Make sure all the gear shift bushings are OK and the coupler is properly adjusted. Likewise the clutch.
New lube can be an improvement but will not change the fact that the synchros are worn. Many 915 owners like Swepco 201.
Also, please try the search button. The subject has been covered many times.
And post pics of your car.
