Manual Gearbox Oil
Gearbox oil is quite different to engine oil, it has a higher viscosity meaning it's thicker than engine oil. As it doesn't have to deal with the same contaminants as engine oil it has less additives and detergents to clean it. It's vital to use the correct gearbox oil grade for your gearbox to get the best performance and longevity out of the parts of the gearbox.
Gearbox Fluid looks dirty
The gearbox fluid looks like it has contaminants on inspection. Fluid can also look discoloured and very different to fresh gearbox fluid.
Gearbox temperature warning light showing on dashboard
There is a light similar to the one pictured here showing up on the dashboard. Different manufacturers sometimes have slightly different warning light symbols but it should be similar to the one pictured.
Gearbox makes grinding sound
When changing gear the gearbox makes a loud grinding noise. This will be a more common occurrence on cars with manual gearboxes but can also happen with automatics. There can be various reasons for it but it always presents as a loud, unpleasant sound of mechanical scraping/grinding.
Gear changes are difficult
Changes between gears have become difficult, the gearstick no longer moves freely between gears, it sometimes seems to get stuck or won't go into a gear properly.