Noises is either bearing wear or gear wear. Using a thicker gear oil may help but it's not the solution. Using a thicker oil may affect the synchro operation. Making the gear changes crunch..this will be more annoying that the gear noise I suspect.

You will need to buy a workshop manual, and strip down the gearbox, and if any of the bearings are making crunchy or 'dry running' noise, then the bad bearings should be replaced. To get a gearbox service business to overhaul your gearbox will cost you 10 times as much.

Good Luck,
Chris