In my opinion, your gearbox needs some serious work. If you have noisy 2nd/4th gears and you have metal in the oil you will never cure it by changing rubbers or oil. I suspect that you have lost case hardening from gear teeth or even have a broken or chipped teeth. If it were only the synchros cones that were damaged you would have brass coloured particles in the oil and it would be just more difficult to change gear. Get a new gearbox or get that one completely overhauled. Can you still get a Spec 2000 box because they were a great improvement on the original box.