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Transmission Subcategories
Only subcategories containing verified fitment products are shown.
Differential oil protects a severe gear contact
Final-drive gears reduce speed and multiply torque before it reaches the wheels. Hypoid geometry allows a compact, low propshaft position but creates substantial sliding across the tooth faces. Oil must form a film and its additives must react protectively before metal surfaces weld or scuff.
The same lubricant cools bearings, carries wear particles to magnets or sumps, prevents rust and may control friction in limited-slip clutches. These duties explain why a plausible viscosity is only one part of selection.
Oil circulation through the axle
- The rotating crown wheel lifts oil from the housing sump.
- Oil is carried into the crown-wheel and pinion mesh.
- Splash and internal galleries feed pinion, carrier and axle bearings.
- Differential gears and thrust surfaces receive the dispersed lubricant.
- Limited-slip plates, gears or actuators operate in the specified fluid.
- Heat transfers into the casing and surrounding air or a cooler.
- Oil drains back while debris settles or is captured by magnets.
Differential and final-drive arrangements
| Arrangement | Lubricated components | Fluid implication |
|---|---|---|
| Open hypoid axle | Crown wheel, pinion, differential gears and bearings. | Needs the specified extreme-pressure gear oil. |
| Clutch limited-slip differential | Gearset plus friction plates and preload mechanism. | Requires controlled friction and possibly approved modifier. |
| Helical/torque-biasing differential | Meshing helical gears and thrust faces. | Do not assume clutch-type modifier is wanted. |
| Locking differential | Gearset and mechanical, pneumatic or electric lock mechanism. | Specification must suit all internal components. |
| Active/electronic differential | Final drive plus controlled clutch packs or actuator. | May use one or more dedicated fluids and adaptation. |
| Transaxle/shared sump | Gearbox, synchronisers, final drive and differential. | Use the transmission specification, not generic axle oil. |
| Separate front/rear final drive | AWD axle unit distinct from transfer case. | Each housing can require a different fluid. |
Understanding viscosity
Multigrade gear-oil labels
A grade such as 75W-90 describes behaviour against SAE gear-lubricant limits at low and high temperature. Gear-oil numbers use a different scale from engine-oil numbers, so a 75W-90 is not literally much thicker than an SAE 10W-40 engine oil by those labels alone.
Cold flow
Low-temperature viscosity affects churning losses and how quickly oil reaches bearings and tooth contacts. An oil that is too thick for the climate or design can increase drag and starve components during initial movement.
Operating-temperature film
High-temperature viscosity supports film thickness under load. Choosing an arbitrarily thicker grade may increase heat, reduce flow and disturb clutch operation. Use the specified grade and approval for normal or severe service unless the manufacturer publishes an alternative.
Performance categories and approvals
| Specification evidence | What it addresses | Limitation |
|---|---|---|
| SAE viscosity grade | Defined low- and high-temperature viscosity ranges. | Does not define full additive or friction performance. |
| API GL category | Broad gear-oil service and extreme-pressure performance. | Does not guarantee manufacturer approval. |
| Vehicle/axle approval | Materials, durability, friction and test requirements. | Exact wording and revision must match. |
| Limited-slip claim | Friction suitability for certain clutch differentials. | Not universal across every LSD design. |
| Product data sheet | Viscosity, approvals and declared applications. | Different from informal “suitable for” language. |
| Service bulletin | Updated oil, capacity or repair procedure. | Must apply to the exact axle and build range. |
API GL-4 products are often associated with moderate-duty gearsets and transmission synchroniser compatibility, while GL-5 performance addresses severe hypoid duty. This is not a simple ladder where the higher number automatically replaces the lower. Additive level, yellow-metal compatibility and friction behaviour must suit the unit.
Extreme-pressure chemistry and materials
Under boundary lubrication, sulphur-phosphorus and other additive systems form sacrificial films on gear surfaces. The chemistry must protect at high load without unacceptable corrosion or friction change. Modern formulations balance oxidation control, foam resistance, demulsibility and seal compatibility alongside wear performance.
Limited-slip clutches need a controlled relationship between static and dynamic friction. Unsuitable oil can create stick-slip chatter or reduce torque transfer. Adding modifier lowers or changes friction and should be done only in the quantity and product approved for the axle. Many oils already contain the intended additive.
Some housings contain bronze thrust washers or other sensitive materials. A broad gear-oil category cannot prove compatibility with every internal alloy. Manufacturer specifications incorporate the intended component set.
Selection checklist
| Check | Possible variation | Why it matters |
|---|---|---|
| Axle code | Ratio, supplier and internal differential. | Model name may cover several final drives. |
| Differential type | Open, clutch LSD, helical, locking or active. | Friction and service procedure differ. |
| Shared/separate sump | Transaxle or independent axle housing. | Determines whether synchronisers share the oil. |
| Exact approval | Manufacturer number and revision. | Primary compatibility evidence. |
| Viscosity | Climate or duty-specific grade. | Controls flow and film at operating temperatures. |
| Capacity/level method | Measured quantity, overflow or diagnostic fill. | Prevents starvation or churning. |
| Service hardware | Plug washer, O-ring, cover gasket and one-time plugs. | Ensures a leak-free refill. |
Limited-slip differential considerations
Clutch LSD plates alternate between members linked to the differential case and side gears. A speed difference makes the clutch pack resist relative motion. Oil friction properties influence smooth engagement, chatter, heat and available locking action.
A helical differential creates torque bias through gear forces and does not necessarily benefit from clutch modifier. An electronically controlled differential may apply wet clutches using a dedicated fluid or shared axle oil and can require a bleed, calibration or adaptation after service.
Tyres of unequal circumference force a differential to work continuously. On AWD vehicles this can also load the centre coupling or transfer unit. Correct tyre size, pressure and similar wear are part of lubricant and differential life.
Service intervals and severe use
Follow the time, mileage and inspection interval for the exact axle. Some schedules describe lubricant as long-life under normal use yet specify replacement after towing, deep-water exposure, track use or repair. “Filled for life” does not mean immune to leakage, contamination or mechanical failure.
Towing, high payload, prolonged high speed and steep gradients increase tooth load and temperature. Off-road water crossings can draw moisture through a hot axle vent as the casing cools. Inspect oil promptly after suspected immersion rather than waiting for milky lubricant to damage bearings.
Condition and fault evidence
| Finding | Possible meaning | Response |
|---|---|---|
| Fine paste on magnetic plug | Normal run-in or gradual wear depending on amount. | Compare with history and service guidance. |
| Chips or large flakes | Gear, bearing or thrust-component damage. | Stop and inspect internally. |
| Milky oil | Water contamination. | Find entry path and assess bearings and gears. |
| Burnt odour/darkened oil | Excess heat, oxidation or clutch distress. | Diagnose load, level and differential condition. |
| Foam | Overfill, wrong oil, air entry or contamination. | Correct level and specification. |
| Whine on drive or coast | Gear contact, bearing preload or wear. | Oil change alone cannot reset gear geometry. |
| Chatter on tight turns | LSD friction issue or mechanical damage. | Verify oil, modifier history and clutch condition. |
Leak sources and ventilation
Oil can escape at pinion and axle seals, cover joints, drain/fill plugs, breathers and casing damage. A blocked vent raises internal pressure as the axle heats and can force oil past otherwise serviceable seals. Cleaning the exterior and observing the highest fresh wet point helps locate the source.
Seal replacement should include checks for shaft grooves, bearing play and vent condition. Overfilling can also push oil out. A wet pinion area may receive engine or transmission oil from elsewhere, so identify the fluid and route before dismantling.
Safe draining and filling
- Confirm oil, capacity, fill level, plug hardware and service temperature.
- Place the vehicle level on rated supports or a suitable lift.
- Clean around both plugs and ensure the fill plug will open first.
- Drain into a clean pan and inspect oil and magnetic debris.
- Renew specified washers, O-rings or plugs and torque the drain.
- Transfer new oil with clean equipment dedicated to compatible fluid.
- Set the level by measured quantity or fill-hole procedure exactly.
- Torque the fill plug and clean residue from the housing.
- Road-test, operate the differential normally and recheck for leaks.
Some active units require diagnostic temperature monitoring, pump operation or multiple chambers. Others have no conventional drain, and cover removal requires a new gasket and approved sealant bead. Never infer the procedure from a nearby transfer case.
Common mistakes
- Buying by viscosity alone without the required approval.
- Treating GL-5 as an automatic upgrade over every GL-4 requirement.
- Adding limited-slip modifier to an oil that already contains it.
- Using axle oil in a transaxle that requires synchroniser-compatible fluid.
- Draining before confirming the seized fill plug can be removed.
- Filling an unlevel vehicle to the edge of the hole.
- Overfilling to cure noise or leakage.
- Using a transfer pump contaminated with another fluid.
- Ignoring metal chips because the oil still looks clean.
- Assuming fresh oil can repair worn gears or incorrect bearing preload.
Upgrades, towing and modified vehicles
Additional cooling or a revised lubricant may be appropriate only where the axle manufacturer or a competent engineer specifies it. Differential covers can change capacity, cooling and internal oil guidance; extra volume does not guarantee that pinion bearings receive correct splash.
After fitting a limited-slip or changing gear ratio, use the builder's break-in and oil requirement while preserving axle compatibility. Modified final-drive behaviour and track use should be disclosed to the insurer, and service intervals should reflect actual temperature and load.
UK safety and environmental handling
Differential condition can affect driveline security and vehicle control, although oil specification is not itself an MOT test item. Serious leaks, insecure components, excessive bearing play or a failing final drive may be assessed under relevant current criteria. Oil reaching brakes or tyres creates immediate danger.
Used gear oil is harmful waste. Wear gloves, prevent discharge to drains or soil and take it to an authorised collection point. Clean spills promptly with suitable absorbent and dispose of contaminated materials correctly.
Differential oil FAQs
Q: What does differential oil lubricate?
A: It protects final-drive gears, differential gears, bearings and applicable LSD parts.
Q: Is 75W-90 the same for every axle?
A: No. Approval, additives and friction properties can differ.
Q: Can GL-5 always replace GL-4?
A: No. Use the exact category and manufacturer specification required.
Q: Does every limited-slip differential need modifier?
A: No. Only use the approved product and quantity for that design.
Q: Can differential oil be used in a manual gearbox?
A: Only when the transmission specification explicitly permits that oil.
Q: Why open the fill plug before draining?
A: It confirms the unit can be refilled before its oil is removed.
Q: Is metal paste on the drain magnet normal?
A: A small fine amount may be, but chips or heavy debris require investigation.
Q: What causes milky differential oil?
A: Water contamination, often after immersion or vent/seal problems.
Q: Can thicker oil stop differential whine?
A: It may mask noise but cannot repair gear or bearing damage.
Q: Should differential oil be changed after towing?
A: Follow the severe-service schedule for the exact vehicle and duty.
Q: Can different gear oils be mixed?
A: Avoid mixing unless compatibility and the final specification are confirmed.
Q: Where should used differential oil go?
A: Take it in a suitable container to an authorised waste-oil facility.
Q: Can a differential leak affect the MOT?
A: Yes where leakage, contamination or driveline condition meets current defect criteria.
