Oils and Lubrication Information
Understanding Automotive Oils and Lubricants
A practical guide to oil viscosity grades, engine-oil specifications, gearbox and differential oils, automatic transmission fluids, greases and the importance of using the correct lubricant.
What does automotive oil actually do?
Oil forms a protective film between moving components, reducing direct metal-to-metal contact, friction and wear. It also carries heat away from heavily loaded areas and helps protect internal surfaces from rust, corrosion, sludge and harmful deposits.
Engine oil must cope with combustion contamination, repeated cold starts, extremely hot turbocharger bearings and long periods of operation. Gearbox and axle oils must protect gears and bearings while also working correctly with synchronisers, clutch packs, seals and electronically controlled components.
The correct lubricant is therefore defined by several factors. These normally include viscosity grade, performance specification, base-oil type, additive package and approval for the particular engine, transmission or component.
Indicates that the oil satisfies defined low-temperature cranking and pumping limits.
Describes the oil's viscosity range when hot. It does not mean that oil becomes thicker as it heats.
Oil Viscosity Grades Explained
Viscosity is a measure of a fluid's resistance to flow. Oil must circulate quickly enough when cold while retaining adequate film strength when the engine or transmission is hot.
Lower Cold Grades
Oils such as 0W-20, 0W-30 and 5W-30 are designed to satisfy lower-temperature cranking and pumping limits, helping the lubricant circulate promptly after a cold start.
Traditional Cold Grades
Grades such as 10W-40 and 15W-40 are commonly associated with older engines and certain commercial or heavy-duty applications, but suitability still depends on the required specification.
Lower Hot Viscosity
Lower operating-temperature grades can reduce internal drag and support fuel economy. They should only be used where the engine manufacturer permits that grade.
Higher Hot Viscosity
Higher hot grades retain greater viscosity at operating temperature. They may be specified for particular engines or conditions, but are not automatically better.
Common Multigrade Engine Oils
These are broad comparisons only. The correct viscosity and performance specification must always be confirmed for the particular engine.
| Grade | Cold characteristics | Hot characteristics | Typical context |
|---|---|---|---|
| 0W-20 | Very low cold-temperature grade for rapid circulation. | Low operating-temperature viscosity. | Modern engines specifically designed for low-viscosity oil. |
| 5W-30 | Good cold-start flow for a wide range of climates. | SAE 30 operating-temperature viscosity. | Widely used in modern petrol and diesel engines. |
| 5W-40 | Similar cold-temperature grade to 5W-30. | Higher hot-viscosity grade than SAE 30. | Selected turbocharged, petrol and diesel applications. |
| 10W-40 | Higher cold-temperature grade than 5W oils. | SAE 40 operating-temperature viscosity. | Common in many older engines. |
| 15W-40 | Less suited to very low temperatures than lower W grades. | SAE 40 operating-temperature viscosity. | Traditional diesel, commercial and older-engine applications. |
| 20W-50 | Relatively high cold-temperature viscosity. | Higher SAE 50 operating-temperature viscosity. | Certain classic or older engines where explicitly recommended. |
Engine-Oil Specifications Explained
The viscosity grade describes how the oil flows. The specification describes the type and level of performance the oil is formulated to deliver.
API S Specifications
API categories beginning with S relate primarily to petrol-engine service. Later categories introduce more demanding tests covering wear, deposits, oxidation, turbochargers and timing-chain protection.
API C Specifications
API categories beginning with C primarily relate to diesel engines. Requirements can include soot handling, wear resistance, oxidation control and compatibility with modern emissions systems.
European Oil Categories
ACEA classifications are widely used for European vehicles. Categories cover petrol engines, light-duty diesels, emissions-compatible oils and heavy-duty diesel applications.
DPF and Catalyst Compatibility
ACEA C-category oils are designed for suitable engines fitted with exhaust after-treatment systems such as catalytic converters and diesel particulate filters.
Vehicle-Specific Requirements
Vehicle manufacturers often publish their own oil specifications. These may add engine-specific tests, extended-drain requirements or restrictions relating to emissions equipment.
Controlled Additive Chemistry
Low- and mid-SAPS oils limit sulphated ash, phosphorus and sulphur to help protect compatible particulate filters and catalytic converters. The exact required specification must still be matched.
Engine oils are available in different viscosity grades, base-oil types and performance specifications.
Understanding Engine-Oil Base Types
The base oil provides the foundation of the lubricant, while additives are blended into it to achieve the required protection, cleanliness and performance.
Transmission and Drivetrain Lubricants
Manual gearboxes, automatic transmissions, differentials and transfer cases can require completely different fluids, even when products appear to have similar viscosities.
Automatic Transmission Fluid
ATF acts as a hydraulic fluid, lubricant, coolant and controlled-friction medium. Its viscosity and friction characteristics affect gear changes, clutch engagement and torque-converter operation.
Gearbox and Differential Oil
Gear oils protect gears and bearings under heavy loading. Manual gearboxes, hypoid differentials, transfer cases and limited-slip units can each have different lubricant requirements.
Manual Gearbox Oil
Manual transmission oil must protect gears and bearings while allowing synchronisers to operate correctly. Some gearboxes use dedicated MTF, gear oil or a specified ATF.
Differential Oil
Differential gears operate under heavy sliding loads and commonly require extreme-pressure lubricant. Limited-slip differentials may require additional friction modifiers.
Transfer-Case Fluid
A transfer case may require gear oil, ATF or a manufacturer-specific fluid. Modern units can contain clutch packs that depend on precise friction characteristics.
API GL-4 and GL-5 Gear Oils
GL categories relate to gear-oil performance and loading. They are not simply a ranking of overall product quality.
Manual Gearboxes and Moderate Loading
GL-4 oils are widely used in manual transmissions and certain transfer boxes or axles. Their additive balance may be selected to provide gear protection while remaining compatible with synchroniser operation.
Highly Loaded Hypoid Final Drives
GL-5 oils provide extreme-pressure protection for highly loaded hypoid gears and are commonly specified for differentials. GL-5 is not automatically a universal upgrade for a gearbox requiring GL-4.
Automotive Greases Explained
Grease combines lubricating oil with a thickener and additives. It is used where liquid oil would escape too quickly or where components require longer-term retention.
General Chassis Grease
Used for suitable hinges, pins, joints and chassis lubrication points. Water resistance and load capacity should be checked.
Wheel-Bearing Grease
Bearing grease must resist mechanical working, heat, water and oxidation while maintaining an adequate lubricating film under load.
Moly Grease
Molybdenum-disulphide grease can provide additional protection under heavy sliding or shock loads but is not appropriate for every bearing or joint.
CV-Joint Grease
Constant-velocity joints normally require dedicated high-load grease formulated for severe articulation, sliding contact and heat.
Semi-Fluid Grease
Semi-fluid grease can flow around internal components while being less likely to escape past worn seals than conventional gear oil.
Specialist Lubricants
Brake components, electrical connectors, rubber seals and threaded fasteners can require specialist lubricants rather than general-purpose grease.
How to Choose the Correct Oil or Lubricant
Work from the vehicle or component manufacturer's requirements rather than selecting oil solely by brand, price, colour or viscosity.
Identify the component
Confirm the exact engine, gearbox, transfer case or differential being serviced.
Check the viscosity
Find the prescribed SAE grade or permitted range for the intended climate and use.
Match the specification
Check the required API, ACEA, GL or manufacturer-specific standard.
Check special equipment
Consider DPFs, catalytic converters, wet clutches, limited-slip differentials and active transfer cases.
Use the correct quantity
Follow the stated filling capacity, level-setting method and replacement procedure.
Follow the service interval
Take account of age, mileage, towing, short journeys, off-road use and other severe conditions.
Oil and Lubrication FAQs
Is 5W-40 better than 5W-30?
Not automatically. The correct choice depends on the engine manufacturer's approved viscosity and full lubricant specification.
Does dark engine oil mean it has failed?
Not necessarily. Detergents and dispersants hold contamination in suspension, and diesel engine oil can darken quickly.
Can thicker oil reduce engine noise?
It may alter noise in some circumstances, but an unapproved thicker oil can delay lubrication or affect hydraulic components.
Can GL-5 replace GL-4?
Not as a general rule. GL-5 axle oil may have unsuitable friction characteristics for a synchronised gearbox requiring GL-4.
Can ATF be used in a manual gearbox?
Yes, but only where the gearbox manufacturer specifies it. Other gearboxes require dedicated MTF or conventional gear oil.
Can different oils be mixed?
Some engine oils are physically miscible, but mixing products can reduce the intended performance and should not replace using the correct oil.
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