9 Products
Your Current Vehicle
Or
Select Your Vehicle
Transmission lubricant is both protection and calibration
Gear teeth experience concentrated sliding and rolling loads, while bearings require a stable film at speed. Synchronisers depend on carefully controlled friction to match shaft speeds. In an automatic or wet dual-clutch unit, the same fluid may apply clutches and transmit hydraulic signals through passages measured in fractions of a millimetre.
This range of duties explains why “oil is oil” is unsafe reasoning. The approved formulation influences efficiency, shift timing, wear, cooling and seal life.
Main lubricant families
| Fluid family | Typical application | Critical characteristic |
|---|---|---|
| Manual transmission oil | Synchromesh gearbox or transaxle. | Synchroniser friction and yellow-metal compatibility. |
| Automatic transmission fluid | Torque-converter planetary transmission. | Controlled clutch friction and hydraulic viscosity. |
| CVT fluid | Belt, chain or specialised continuously variable unit. | Pulley contact, pressure control and anti-wear performance. |
| Wet dual-clutch fluid | Shared clutch, gear and mechatronic circuit. | Wet-clutch friction plus gear and bearing protection. |
| Dry dual-clutch gear oil | Mechanical gear train separate from dry clutches. | Gear and actuator system may require different fluids. |
| Transfer-case fluid | All-wheel-drive distribution unit. | Chain, gear and controlled-clutch requirements. |
| Final-drive oil | Differential and hypoid gears. | Extreme-pressure protection and possible limited-slip friction. |
Viscosity grades and approvals
Viscosity describes flow resistance
SAE gear-oil grades use a different numerical scale from engine-oil grades. A 75W-90 gear oil is not necessarily thicker than an engine oil merely because its numbers are larger. Multigrade notation indicates behaviour at low and high reference temperatures, not quality or application approval.
Approval defines a performance package
A manufacturer specification can include viscosity, oxidation, friction, foaming, wear, corrosion and seal tests for a particular transmission family. Two products with the same nominal viscosity may behave very differently in synchronisers or clutches.
API categories need context
GL categories describe broad gear-oil performance but do not override vehicle requirements. Higher numbers are not automatic upgrades. An extreme-pressure package suitable for a hypoid axle can impair synchroniser operation in a gearbox not designed for it.
Exact selection evidence
| Evidence | What it identifies | Risk of ignoring it |
|---|---|---|
| VIN equipment record | Factory transmission and driveline variant. | One model may carry several unrelated units. |
| Gearbox code or data plate | Transmission family and revision. | Fluid and fill method may differ by suffix. |
| Owner or workshop information | Approval, capacity and service procedure. | Generic catalogue assumptions can be wrong. |
| Production date | Mid-cycle lubricant or hardware change. | Old and revised fluids may have different compatibility. |
| Current technical bulletin | Official supersession or updated fill. | Obsolete product may need a defined replacement. |
| Fluid container approval | Whether the product actually claims required specification. | Marketing phrases alone do not prove compliance. |
| Driveline configuration | Transfer case and differential equipment. | Several reservoirs may sit close together. |
Friction requirements in manual gearboxes
A synchroniser uses friction between a cone and ring to bring rotating components to similar speed before engagement. Oil that is too slippery, too viscous when cold or chemically unsuitable can create slow, baulky shifts. Worn cones cannot be restored by a fluid advertised as a cure.
Some manual gearboxes specify an ATF-like fluid or dedicated low-viscosity oil rather than traditional gear oil. Follow the approval even when the choice seems unusual. Differential gears may share the same sump and affect additive needs.
Hydraulic and clutch control
Automatic transmission fluid passes through pumps, solenoids and calibrated valve-body orifices. Viscosity changes pressure losses and fill times, while friction modifiers control how clutch plates engage. A mismatched fluid can cause harshness, flare, shudder or excess heat without any external leak.
CVTs and dual-clutch transmissions have distinct contact mechanics. A broad multi-purpose product should be used only when the required approval is explicitly and credibly met. Never add a limited-slip axle additive to a transmission unless the manufacturer specifies it.
Temperature and fluid ageing
Heat accelerates oxidation, thickens degraded residues and weakens additives. Towing, mountain driving, stop-start use, track work, a restricted cooler and clutch slip increase thermal load. Repeated overheating is a fault to diagnose, not simply a reason to change oil more often.
Water contamination promotes corrosion and changes friction. It can enter through a submerged breather, failed heat exchanger, condensation or poor storage. Milky fluid and rust particles require identification of the route before refilling.
Service methods and their limits
| Method | What it renews | Boundary |
|---|---|---|
| Manual gearbox drain and fill | Most freely draining sump oil. | Pockets and galleries may retain a quantity. |
| Automatic pan drain | Oil in the pan and accessible passages. | Converter and cooler retain fluid. |
| Filter and pan service | Drainable oil, filtration and magnet cleaning. | Does not exchange every litre. |
| Repeated drain and refill | Progressively dilutes old fluid. | Must use approved sequence and level setting. |
| Machine exchange | Potentially a larger fluid proportion. | Not approved for every unit or failing transmission. |
| Differential refill | Oil in the axle or final-drive casing. | Limited-slip and drain position change the method. |
Safe drain-and-fill preparation
Obtain the correct lubricant, capacity range, plug locations, torque values, seal parts and level instructions before lifting the vehicle. Some drain and fill plugs look alike; others sit near engine, transfer-case or cooling-system drains. Confirm by diagrams.
Make sure the fill plug opens before removing the drain plug. A seized filler discovered after draining leaves the vehicle immobile. Clean around openings so road dirt cannot enter, and use the exact tool to avoid rounding shallow internal drives.
General service sequence
- Confirm transmission identity, approval, fill method and relevant service interval.
- Record leaks, noises, shift complaints and diagnostic codes before changing the evidence.
- Warm the fluid only as instructed, then support the vehicle securely and level.
- Open the verified fill point, clean it and position a suitable drain container.
- Remove the drain plug carefully and inspect the fluid, plug magnet and measured quantity.
- Renew the correct washer, O-ring or plug and tighten to specification.
- Fill with clean dedicated equipment so products are not cross-contaminated.
- Run, circulate through selections or rotate the driveline only as the procedure states.
- Set the final level at the required temperature and operating condition.
- Road-test safely, recheck for leakage and record the product and quantity used.
Level systems
| Level method | Required condition | Common error |
|---|---|---|
| Side filler opening | Vehicle level, oil at specified edge condition. | Filling while the vehicle is tilted. |
| Dipstick | Specified temperature, engine state and selector position. | Reading the wrong scale or immediately after filling. |
| Standpipe overflow | Exact fluid temperature and pump operation. | Assuming overflow always means correct level. |
| Measured dry fill | Only after defined overhaul with known empty circuits. | Using dry capacity for an ordinary drain. |
| Electronic guided fill | Scan data and staged procedure. | Substituting coolant temperature for transmission data. |
Reading drained fluid
Colour changes with dye, oxidation and suspended material, so it is only one clue. Smell, clarity, water, particles and deposit quantity add context. A light grey paste on a magnetic plug can reflect fine wear; sharp flakes, teeth fragments, bronze material or heavy black clutch residue are not corrected by fresh oil.
Send a representative clean sample for laboratory analysis when trend monitoring justifies it. A sample from a dirty drain pan or solvent-washed plug is not reliable. Keep suspected wrong or contaminated fluid separate from other waste.
Mixing, flushing and additives
Even when two products meet the same approval, mixing is best minimised because the remaining package and contamination are unknown. Products with different intended specifications should not be blended to achieve an average viscosity.
Aftermarket friction modifiers, stop-leak agents and cleaners change a calibrated formulation. Use them only when the vehicle or lubricant manufacturer explicitly approves the exact application. A leak needs a seal or housing diagnosis; noise needs mechanical investigation.
Fault symptoms and urgency
| Finding | Possible fluid relationship | Response |
|---|---|---|
| Leak or falling level | Seal, plug, cooler or casing fault. | Repair promptly before lubrication or pressure is lost. |
| Cold shift baulk | Viscosity or synchroniser friction issue. | Verify approval and diagnose wear. |
| Automatic shudder | Degraded or incorrect friction performance. | Check codes and transmission condition before fluid decisions. |
| Whine or rumble | Low oil, aeration or bearing/gear damage. | Stop if severe; check level and mechanical state. |
| Burnt odour | Overtemperature or clutch slip. | Find the heat source; oil alone is not a repair. |
| Milky appearance | Water or coolant contamination. | Do not operate until the entry path is identified. |
| Loss of drive | Severe low level, pressure or internal failure. | Stop and recover the vehicle. |
Storage and environmental handling
Keep unopened containers dry, upright and labelled. Once opened, exclude dust and moisture and do not pour unused fluid back from a workshop jug. Dedicated pumps prevent a small residue of incompatible oil contaminating a sensitive transmission.
Used oil must go to an authorised recycling route. Do not pour it into drains, onto soil or mix it with coolant, brake fluid or solvents. Clean spills immediately because lubricant on tyres, brakes or the road creates a serious hazard.
Gearbox oil and transmission fluid FAQs
Q: Can I choose gearbox oil by viscosity alone?
A: No. The exact manufacturer approval and transmission code are also essential.
Q: Does red fluid always mean ATF?
A: No. Colour is only dye and does not prove specification or condition.
Q: Can manual gear oil be used in an automatic?
A: No unless the manufacturer explicitly specifies the same approved product, which is unusual.
Q: Are CVT and automatic fluids interchangeable?
A: Not generally; their friction and pressure-control requirements differ.
Q: Is a higher GL category always better?
A: No. Additive chemistry must suit the gears and synchronisers in the unit.
Q: Why open the fill plug before draining?
A: It confirms that the transmission can be refilled before its oil is removed.
Q: Can I refill only the quantity that drained?
A: It is a useful check, but final level must follow the specified procedure.
Q: Why does transmission temperature matter?
A: Fluid expands as it warms, changing the level in temperature-based systems.
Q: Is metal paste on a drain magnet normal?
A: A modest fine paste may be, but flakes, chunks or heavy deposits require diagnosis.
Q: Will fresh fluid repair worn gears or clutches?
A: No. It can support correct operation but cannot replace damaged material.
Q: Should an old transmission be machine-flushed?
A: Only after assessing its condition and confirming the method is approved.
Q: Can I add a stop-leak product?
A: Avoid unapproved additives; identify and repair the actual leak.
Q: When must I stop driving?
A: Stop for rapid leakage, loss of drive, severe noise, overheating or confirmed water contamination.
