Gearbox Oil: How to Check the Level and Change It

Engine oil gets changed at every service, but gearbox oil is often forgotten until gear changes become notchy, noisy or slow. Low or worn-out gearbox oil is one of the main causes of expensive gearbox repairs, and checking it is easier than most people think.

This guide explains what gearbox oil does, the different types, how to check the gearbox oil level and how to change it.

What Does Gearbox Oil Do?

Gearbox oil (also called transmission oil, gear oil or, in automatics, transmission fluid) does several jobs:

• Lubricates gears, bearings and shafts so metal doesn’t grind on metal.
• Protects against wear under very high pressure between gear teeth.
• Cools the gearbox by carrying heat away.
• Helps synchronisers work in manual gearboxes, so gears engage smoothly.
• In automatics, works as a hydraulic fluid that actually operates the clutches and bands that change gear.

Types of Gearbox Oil

Manual gearbox oil

Usually a thicker oil, such as 75W-80, 75W-90 or 80W-90, with an API GL-4 or GL-5 rating. GL-4 and GL-5 aren’t always interchangeable: GL-5 has more extreme-pressure additives, which can attack the brass synchronisers in some manual gearboxes. Some manual gearboxes, including certain Hondas and some European models, use a special fluid or even ATF.

Automatic transmission fluid (ATF)

A thinner, red or amber fluid with friction modifiers. There are many specifications (Dexron, Mercon, Toyota WS, ZF Lifeguard, Mercedes 236.x and others), and using the wrong one can damage an automatic gearbox.

CVT fluid

Continuously variable transmissions (common in Nissans, Hondas, Toyotas and Subarus) use a specific CVT fluid. Never use ordinary ATF.

DSG/dual-clutch fluid

VW and Audi DSG gearboxes use a special DCT fluid. Wet-clutch DSGs need regular fluid changes, often every 60,000 km.

Differential and transfer case oil

Rear-wheel-drive and 4×4 vehicles, such as the Toyota Hilux and Ford Ranger, have separate oil in the differentials and transfer case, often a GL-5 80W-90 or 75W-90. Limited-slip diffs may need an LSD additive.

Your owner’s manual or service book lists the exact specification. Engine oil like 5W-30 is for the engine, not the gearbox.

Signs of Low or Worn Gearbox Oil

• Notchy or stiff gear changes, especially when cold.
• Grinding when changing gear (also a sign of a worn clutch or synchros).
• Whining or humming that changes with road speed.
• Automatics: slipping, delayed engagement when selecting Drive or Reverse, harsh or flaring shifts, or a transmission warning light.
• Oil spots under the car, at the joint between engine and gearbox or under the gearbox.
• A burning smell: overheated fluid.
• Dark, burnt-smelling ATF instead of clean red or amber.

How to Check Gearbox Oil Level

Manual gearboxes

Most manual gearboxes have no dipstick. The level is checked at the filler plug on the side of the gearbox.

1. Park on level ground. If you need to lift the car, lift it level on axle stands or a ramp. Never work under a car supported only by a jack.
2. Find the filler (level) plug, on the side of the gearbox casing. The drain plug is lower down, at the bottom.
3. Always loosen the filler plug first. If you drain the oil and then can’t open the filler plug, you’re stuck.
4. Remove the filler plug. The oil should be level with the bottom of the hole, or just dripping out. Check with a bent finger or a cable tie.
5. If it’s low, top up with the correct oil using a squeeze bottle or pump until it just starts to run out.
6. Refit the plug (with a new washer if specified) and tighten it to the correct torque.

Automatic gearboxes with a dipstick

1. Warm the car up to normal operating temperature.
2. Park on level ground with the engine running and the handbrake on (on most cars; check your manual).
3. Move the selector slowly through all positions, then back to Park.
4. Pull the transmission dipstick (often yellow or red), wipe it, reinsert it fully and pull it out again.
5. The fluid should be between the HOT marks. Note the colour and smell.
6. Top up through the dipstick tube with the correct ATF, a little at a time. Overfilling is as bad as underfilling.

Sealed automatic gearboxes

Many modern automatics have no dipstick. The level is checked through a plug, at a precise fluid temperature, using a diagnostic tool. This is a workshop job.

How to Change Gearbox Oil (Manual)

What you need: the correct oil (usually 1.5–3 litres for a car, more for bakkies), a drain pan, a socket or hex key to fit the plugs, new plug washers, a pump or squeeze bottle, and gloves.

1. Drive for 10–15 minutes so the oil is warm and flows freely, but not scalding hot.
2. Lift the car level and secure it on axle stands or ramps.
3. Loosen the filler plug first.
4. Place the drain pan and remove the drain plug. Many drain plugs are magnetic, so wipe off the fine metal paste. Fine grey sludge is normal; chunks or chips are not.
5. Let it drain completely, then refit the drain plug with a new washer and torque it.
6. Pump new oil into the filler hole until it starts to run out.
7. Refit the filler plug, clean up, and check for leaks after a short drive.

Dispose of used oil at a service station or recycling depot. Don’t pour it down the drain.

Changing Automatic Transmission Fluid

Automatic fluid changes are best done by a workshop with the right equipment:

• A drain and refill replaces only about 30–50% of the fluid. Several changes may be needed.
• A machine flush replaces almost all of it.
• Many gearboxes also have a filter or pan gasket that should be replaced.
• The level must be set at the correct temperature, often with a diagnostic tool.

On a high-mileage automatic that has never had a fluid change, ask a transmission specialist before flushing. Sometimes a change on a neglected gearbox reveals existing wear.

How Often to Change Gearbox Oil

Many manufacturers say manual gearbox oil is “filled for life”, but most mechanics recommend changing it every 60,000–100,000 km, and more often for towing, off-road use or hot climates. Automatic fluid is often changed every 60,000–80,000 km, and DSG fluid every 60,000 km. Always check your service book.

Common Causes of Gearbox Oil Leaks

• Driveshaft (CV) seals, where the drive shafts enter the gearbox.
• Input shaft seal, which shows as oil between the engine and gearbox, and can contaminate the clutch.
• Gear selector seals.
• Loose or damaged drain and filler plugs.
• Damaged gearbox casing from rocks or bad roads.

Fixing small leaks early is much cheaper than replacing a gearbox that ran dry. If yours is past saving, browse gearboxes for sale on Car-Parts.co.za, or browse by make, such as Toyota drivetrain parts.

Frequently Asked Questions

Can I use engine oil in a gearbox?

No, unless the manufacturer specifies it. A few older gearboxes did, but most need a specific gear oil or ATF.

What happens if gearbox oil is low?

Gears and bearings wear faster and run hotter, synchros fail, and an automatic can slip and burn its clutches. Eventually the gearbox fails.

Can I mix different gearbox oils?

It’s best not to. Top up with the same specification. If you don’t know what’s in the gearbox, drain it and refill with the correct oil.

How much does a gearbox oil change cost?

Usually a few hundred rand for the oil for a manual, plus about half an hour of labour. Automatic and DSG fluid services cost more because of the fluid price and the equipment needed.