A gear box is made up of moving parts. When two moving metals are in contact, friction is caused, which results in wear and tear. Lubricants are designed to prevent this damaged caused by the metals in contact.
Just like an engine oil, the gear oil must be able to withstand high temperatures. In some gearbox oils, “extreme pressure” additives are added, which acts as solid lubricants at extreme temperatures.
Generally, the level of gearbox oil should not drop noticeably between routine gearbox oil changes. It is a good practice to get the gearbox oil changed after about 50,000 km. Make sure that the car stands on level ground when checking the gearbox oil level. Most cars have a filler plug (nut) on the side of gearbox which act as oil level indicator. Granted, some cars have a gearbox dipstick. If you notice that the level of oil has dropped significantly; checking its leakage will most likely provide an answer..
The dipstick has marks on it which indicate the upper and lower oil levels. To check the level of gearbox oil of you car, gently remove its dipstick, wipe it clean, place it back in and remove again to read oil level. If oil level is low, refer to owner’s manual and top it up with appropriate grade of oil. A word of caution, though, do not overfill the gearbox oil.
Draining The Gear oil:
Make sure your car has been standing for a few hours to make sure its gear oil has settled to the bottom. Make sure the car is on a leveled ground. Jack it up safely and place axle stands at the front and rear of the car for its support. Lie underneath the gearbox and you will notice a screw to drain gearbox oil. To unscrew this drain, you will need an open spanner, an Allen key or a square ended key, depending upon the type of car. You may also need to remove front tyre of your car as well. Before unscrewing the drain, place a container underneath so that the gear oil doesn’t spill.
Once the drain nut is unscrewed completely, oil will begin to drain. You will need to unscrew filler plug (nut) in order to allow air to pass and make the oil drain, properly. In cars like Toyota Corolla, filler/level plug (nut) of gearbox can be unscrewed from underneath the car. In some cars, oil filler plug (nut) can also be used as an oil level indicator. Some gearbox have a baffle plate and hence you will need to unscrew two drain plugs.
After gearbox oil has been drained, you will need to screw the drain nut back tightly. You can place a funnel to ensure new gear oil you are about to put doesn’t spill. Gently pour new gearbox oil through the filler plug (nut) area.
Tightly screw back filler plug (nut) and wipe this area clean. Make sure you refer to Owner’s manual for the proper grade of gearbox oil.
Furthermore be very cautious of leaks as they are usually found in round damaged gaskets, damaged oil seals or sealing washers. A leak at the filler plug (nut) or drain plug (nut) is probably due to a damaged sealing washer. Fitting a new one should cure the leak.
After making sure there are no leaks and tightening the drain nut and filler plug (nut), gently jack down the car. Drive around for a while and you should feel the gears go soft.