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Gear Oil

Gear oil explained

Designed for far more than straightforward vehicle use, gear oils supplied to the industrial and commercial sectors perform a variety of functions within the transmission system of cars, machinery and high precision equipment. Selecting the right gear oil not only minimises friction over time but maximises load bearing capability and acts well under extreme pressure.  Always refer to the manufacturers recommendations handbook or an oil company registration look up. 

The same 4 levels need to be considered. 

  • Level 1: Mineral, Semi-Synthetic or Fully Synthetic
  • Level 2: The correct viscosity eg 75W-80
  • Level 3: The API, GL-4 or GL-5
  • Level 4: The manufacturer specification of stated

What is gear oil?

Gear oil is a base oil, either mineral, synthetic or part synthetic, and a selection of additives that are blended to produce performance related results. A mineral based oil will generally contain less additives and be more cost effective, whereas a synthetic oil will have more additives and therefore increased performance capabilities but also a higher cost. A part synthetic oil exists between the two, fulfilling both additive and price based requirements.

Additives are combined with the base oil during the production process. When considering which gear oil is best, you should consider the three main functions:

Thermal Stability – When operating at high temperatures, gear oil must remain thermally stable to reduce the build up of sludge and to maintain efficiency. A gear oil that cannot withstand the temperature of the application it is used in, will decrease in power and lead to wear and tear occurring much sooner.

Extreme Pressure – Extreme pressure (EP) additives, or anti-scuff additives as they are also known, help to protect the gearbox in situations where extreme pressure, like heavy front loading or shock loading, will occur.

Contamination and Corrosion – To combat any contaminants that may enter the system, gear oil will need to contain anti rust, anti oxidation or anti foaming agents. These additives perform best in environments with high speeds and low loads or where fluids are commonly used.

*When selecting a gear oil, consider the base oil, desired outcome and viscosity rating required.


As with most other lubricants and oils, like engine oil and hydraulic oil, your selected gear oil will need to have a viscosity rating matching the application it is intended for. The viscosity, in simple terms, refers to the thickness of the oil, or rather its ability to remain stable in certain temperatures. An oil with a low viscosity is more resistant to thickening and operates well in high speed gearboxes with lower loads. A high viscosity oil is designed for use in parts that experience more intense pressure and heat. Manual gears create friction which, in turn, creates heat, as does pressure from heavy loading. Gear oil reduces this friction but the exact viscosity rating required will vary according to the exact application. 

Viscosity Chart

1. Consult gear, bearing and lubricant suppliers if a viscosity grade of less than 32 or greater than 3200 is indicated.

Review anticipated cold start, peal and operating temperatures, service duty and range of loads when considering these viscosity grades.

Select the viscosity grade that is most appropriate for the anticipated stabilised bulk oil operating temperature range.

Baseline stabilised bulk oil operating temperature and bearing lubrication requirements.

2. This table assumes that the lubricant retains its viscosity characteristics over the expected change interval.

3. Determine pitch line velocity of all gear sets. Select viscosity grade for critical gear set taking into account cold startup conditions.

For spur, helical and bevelled enclosed gears.

For oils with VI=90 (recommendations are empirical)


Gear oil is graded according to several factors. The base grade is defined by a GL number and communicates the underlying properties:

  • GL1 – GL3 > Indicates a basic oil unlikely to cope with industrial functions like extreme heat 
  • GL4 > The most common grade containing good amounts of extreme pressure additives. Usually specified for gearbox use. 
  • GL5 > A high performance grade designed for the rigours of heavy lifting and excess friction. Specified for differential use.
  • LS > Limited Slip differential application.

Further to the base oil grade, see SAE J306 viscosity classification (revised SAE J306 effective Feb 2019) gear oil is also graded by the Society of Automotive Engineers (SAE) for gear oils used primarily in vehicles, and by The International Standards Organisation (ISO) for gear oils used in industrial equipment. 

SAE only uses grading numbers of 60 and above for gear oils. These grades follow a pattern whereby single numbers are used to identify monograde gear oils with a higher viscosity rating; for example SAE 90. Gear oils with a lower viscosity rating include a W to indicate they are for use in colder conditions and have a lower viscosity.

Multigrade gear oils that meet multiple grades can be recognised by an initial number indicating performance at 0 degrees, followed by a W and another number showing performance at 100 degrees, eg. 75w90. 

ISO grades are seen more predominantly in gear oils used for industrial purposes. As with SAE grades, the higher the number, the higher the viscosity. 

Gear oil uses

  • With gears being a major component in a vast amount of machinery and industrial hardware, gear oil is used across many industries including construction, manufacturing and, of course, automotive. Gears come in several shapes and sizes, performing a vast array of functions and used for applications including:

    • Conveyors
    • Turbines
    • Propellers
    • Mixers
    • Crushers
    • Hoists and winches

Benefits of gear oil

Using a high quality gear oil and choosing the correct grade will target several key functions and tackle common challenges:

Reducing friction – owing to the heat and pressure sustained in a gearbox, excess friction can occur leading to damage and shorter lifespan

Rust prevention – contaminants like water, particularly in environments where fluid is present can lead to rust and corrosion. A rust and corrosion additive helps to minimise this.

Cooling – By cooling the gearbox, the system can operate at a maximum and again, keep wear and tear to a minimum

Increased output – Using a gear oil for optimum performance will reduce the likelihood of repairs and costly downtime. Additionally, caring for gearboxes can improve longevity and save on unnecessary replacement parts.


The right choice

With so many things to consider, it’s a great idea to talk to someone with the right know-how before selecting a gear oil. Our team of advisors have a wealth of technical knowledge and are always happy to help. 

Alternatively you can find further advice in our partner pages.

Certas Lubricants distributes high quality oils and lubricants from global manufacturers including Shell, Valvoline, Q8Oils, HyperDrive and Castrol. 

Our Supplier Partners

Gear oil FAQs

Gear oil grades are a set of technical specifications that determine the use and viscosity of gear oil. They fall into two categories, those that are primarily for vehicle use and are prefixed by SAE for the Society of Automotive Engineers and those with ISO at the beginning, relating to the International Standards Organisation. Each rating will include either a single number or a series of numbers to indicate the properties of the individual gear oil.

Most original equipment manufacturers (OEM) will list the correct gear oil in the manufacturers manual, making choosing your gear oil a straightforward process. For industrial applications or instances where a manual is not available, the purpose, output and temperature that the gear oil will be used in will need to be carefully considered and checked against the gear oil grade. If in doubt, always seek advice.

Using a gear oil not only increases the overall output by maximising gear box capabilities, but prevents common problems from arising or causing early onset of typical damage. Contamination, corrosion and friction can all reduce the efficiency of a gear box. Using a gear oil to target these problems will result in a better total cost of ownership due to less frequent repairs or replacements.

Which gear oil you choose will depend on the use and the equipment, vehicle or machinery it will be used for. Typically, a synthetic oil has more additives which therefore help with more functions like load bearing and heat resistant. However, a mineral oil has a naturally lower viscosity and so may be a more cost effective solution for less extreme uses. 

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