Transmission fluid or gearbox oil may be new to your ears, even if you've owned a car for years. It could be that it has been a while since you've last changed it or, worse, you may have never done it ever since. 

The gearbox is a component that is often overlooked, and you will probably continue to ignore it until it starts giving you problems in shifting or engaging. It requires regular transmission servicing much like any other car part. This article will guide you step by step on the proper way to change your car's transmission or gearbox fluid.

What is a transmission service? And what's the difference between transmission service and transmission flush?

Transmission servicing is a regular servicing where you change - or have the servicing garage personnel change - the transmission fluid or automatic transmission fluid (ATF) of your car and the filter of those cars that have filters. In comparison, transmission flush is a complete draining of all transmission fluid.

Transmission service (also referred to as transmission fluid change) only involves draining of the used fluid in the pan, which is approximately a third of the oil. On the other hand, transmission flush drains all the transmission fluid, including the oil in the torque converter (almost 40% of the total) and the cooler lines; thus, it's a more intense and vigorous exercise. 

Obviously, a change of 100% of the oil sounds healthier than replacing just a third. However, a good number of automatic transmissions, especially those with filters don't need 100% of the fluid changed. Unlike engine oil, which traps by-products of engine combustion, gearbox fluids operate in sealed units where no external product finds its way. 

As a result, the gearbox oil doesn't require frequent changing like the engine oil; one-third fluid change is sufficient. The one-third change is similar to topping (replacing oil) a cup of coffee (ATF) at a restaurant to keep it hot and fresh. The main difference is that ATF has to be poured out (drained) to make room for the replacement oil. 

Why is there a need to change the gearbox fluid?

Even though the gearbox operates in a sealed unit with no contaminants, there are several reasons why changing the oil may be necessary, such as:

• When the gear oil ages from prolonged usage, its viscosity destabilises, leading to inadequate lubrication. Poor lubrication prematurely damages the gearbox.
• When the transmission system has leaks, it results to low gear fluid and probably contamination, servicing will be necessary to repair the leakage and replace the contaminated oil. 
• When foreign fluids find their way into the transfer case or differential components and contaminate the gear oil, this will require replacing of the transmission fluid. A classic example would be when a car gets submerged for long in the water, such as in a flood.
• When the gear oil starts producing a burning smell and the clutch begins making grinding noises, or the gears start slipping, these are signs that your car has faulty gearbox oil. 

How often should I change the transmission fluid?

How regular or often you should change your transmission fluid depends on the type of oil you use and the model of your car. Typically, synthetic gearbox oil lasts longer than the mineral gearbox oil. On the other hand, different car manufacturers have various recommendations regarding the frequency of changing gear oil. 

Manuals usually recommend gear fluid servicing after approximately 48,250 kilometres (30,000 miles) or after two years and transmission flushing after 80,500 to 96,500 kilometres (50,000 to 60,000 miles) or after three years, whichever comes first. However, the majority of the vehicles hit up to 129,000 kilometres (80,000 miles) without the need of gear fluid change. 

Can I change the gearbox oil on my own?

Yes, changing the gearbox oil is not rocket science. Anyone can do it by following the following simple steps. 

Steps for Changing the Transmission Fluid

• First, acquire the right/recommended gearbox fluid and ensure that you have enough quantity. If your vehicle has a transmission fluid filter, also purchase a replacement filter. For cars without an old gearbox-oil drain plug that requires the opening of the whole reservoir pan, a new oil pan gasket is also recommended. 
• Make sure your car is on a level surface and is warmed up such that the transmission is at average operating temperature. Put the gear on parking or neutral for manual vehicles and pull the handbrake. Do not forget to have a container such as a bucket to collect the used gearbox oil.
• The majority of the modern vehicles have a drain plug through which you can quickly drain the fluids without opening the pan reservoir. Located the drain plug and unscrew it to pour out the old oil. Tapping the old transmission-fluid may take up to thirty minutes, so be patient and wait until the last drop. After draining the oil, secure the drain plug back and tighten it firmly. If your vehicle is among the models with transmission oil filters, do not forget to replace it before adding new transmission fluid. 
• In other vehicle models, there are no drain plugs you have to unbolt the pan reservoir. Loosen the bolts holding the pan in place, loosen one side of the container more than the other so that it tilts collecting the oil to the one end. Drain the oil from the lowered corner, and be careful not to make a mess. The reservoir holds anywhere between 3 to over 10 quarts depending on the vehicle model and size. 
• After draining the oil, unscrew the pan entirely and remove the gasket. Replace the gasket with a new one. Make sure not to scratch or bend the pan's gasket surface. Otherwise, the pan-reservoir won't tightly seal, and you will most probably have leaks.
• Remove and replace the old transmission-fluid filter with a new filter. Clips or bolts hold the filters in place depending on the model. Take careful note of all the O-Rings or others seals and fit them as they were before unbolting the parts. If the transmission-filter has a long intake neck, gently push the neck into place without unseating the O-Ring.
• Inspect the oil pan for any damages. Also, check the type of residue that collects at the bottom after draining the oil. Fine grey clutch dust is a typical residue, but fine metal shaving is a sign of transmission damage. It indicates some parts of the transmission have been grinding and wearing each other out. In some vehicle models, a magnet is fitted at the bottom of the pan to trap the metal shavings.
• Clean the oil pan thoroughly with a solvent, removing all the oil. Use a magnet to remove the metal shaving residues. If the reservoir has a trap-magnet, it will have collected all the metal shavings. Clean it to remove all the metal shaving. Do not forget to fix back the magnet after cleaning.
• After cleaning the pan, position the new gasket ensuring it aligns with pinpoint accuracy. All the four bolt holes should help you to align the gasket.
• Fit back the pan with gasket aligning it properly, ready for bolting. Bolt the pan-reservoir in a crisscross pattern by hand and only use a torque wrench when you are sure all the bolts are well aligned. Bolting by hand prevents the pan or the gasket from being damaged. If you can no longer tighten the bolts manually, you need to use a torque wrench next to tighten them to the manufacturer recommended ft-lbs, ensuring there will be no fluid leaks.
• The system is now ready for new transmission oil. Most vehicles have transmission oil refill cup above the gearbox, and you may require a pipe/tube to transfer the transmission-oil. Suspend the container above the refill cup, connect the tubing and siphon to initiate the flow. The refill is usually slow, taking about half an hour to one hour, so a thin tube is ideal. Regularly check the oil level and end the refill when the oil reaches the required volume. 
• After closing back the refill cap, restart the engine and let it idle for a few minutes. Then change the gear into a different position from "Park" or "Neutral" momentarily and shift back to "Neutral." Recheck the transmission oil again and whether there is any leak. If you find leak immediately, seal the hole. Add more transmission oil if it is below the recommended level. If the level is okay, you're good to go. In case you are not sure about something, consult the experts before you attempt anything.