The gearbox is a very robust item, that suffers untold abuse from many drivers.
Only use engine oil in the gearbox. Heavy EP80 or EP90 will overwork the syncromesh, wearing it out. Putting in thicker oil will accelerate the wear you are trying to hide. Use STP instead, (engine, not gearbox)
If you can hear a rumbling sound with the car ticking over, (idling rpm,) push in the clutch. If it stops, you have noisy gearbox bearings. They may not be too worn, but will need renewing soon. If the rumbling sounds more like grating, they are worn. By pushing in the clutch, you allow the gears to stop turning.
Note... a low oil level in the gearbox will give a noise like worn bearings, topping it up will stop it.
A sloppy gear lever will be a missing, or broken up, nylon cup on the ball at its base. The cost 95p each. Do try to get the old bits out from under the lever, they lay in the oil drain channel, you will need a torch.
A 'zizzing' gear lever is a worn gearbox. This is most often on the over-run. If the base of the gear lever is fry, where it rotates to change gear, grease will sometimes cure the 'ziz'. Get a gearbox kit and rebuild it.
Clean oil coming out of the bell housing hole, with the split pin, is a gearbox oil leak. New seal needed. Dirty oil is either A) gearbox oil that has picked up clutch dust, or B) more likely engine oil. There WILL be a small leak from the engine on high mileage cars. This is normal.
The reversing lamp does not work. First check the bulb, clean the contacts of the bulb holder, check its earths inside, THEN go to the gearbox switch. Most non working reversing lamps are dirty buld holders, dirty earths, or blown bulbs.
Reverse lamps switch is on the side of the gearlever housing. Check the wires are clean with good contacts. They have often fallen out, or grown green with age. If it still does not work, check the power is getting to the switch, (use a bulb.) If all else fails, fit a new switch, (you can get them apart, to clean the insides, but the bakelite insulator breaks up when you try to re-rivet it again.)
Gear level knob keeps coming off. Put some filler in the hole, then screw it on! Leave it to harden, fault cured.
Loud knocking noise when first/reverse is selected, and you move off. A gear tooth is missing on the layshaft!
Gearbox graunches on changing down from top to third or third to second means an old worn out synchromesh. A common fault, learn to double-de-clutch, or rebuild the gearbox. The third to second synchromesh is infamous for only lasting 6000-7000 miles. Live with it, double-de-clutch.
Difficulty in getting first gear stationary. Quite normal, it is baulking. Try selecting second then first, it often works. You have been driving too many modern cars with all synchromesh gearboxes. Welcome back to a non-constant mesh, with synchromesh on only the top three gear, gearbox.