The ultimate guide to gear oil

It’s important that you don’t use gear oil as power steering fluid. The two lubricants have different formulations, and thus shouldn’t be used interchangeably, or you may harm the components and their performance.

Some automatic transmission fluids (ATFs) can be used as power steering fluid, but this will be clearly stated by the manufacturer, so never assume your transmission or gear oils can be used to lubricate power steering systems.

If you’re looking for a gear oil that will protect your gearbox and/or transmission, you need to make sure your oil has the correct grade. While this can be found out via TotalEnergies Lub Advisor or your owner’s manual, it’s worth understanding the meaning behind and differences between gear oil grades.

What are gear oil grades?

Conventional gear oil grades are what you will encounter when shopping for gear oil. The two most common of these are the SAE and ISO grades, with SAE for automotive and ISO for industrial applications.

SAE-grade gear oils

SAE grades are used to label all sorts of lubricants, but for gear oils, only SAE numbers of 60 or above are used.

Like monograde engine oil SAE grades, monograde SAE gear oil grades use a single number, with gear oils for cold seasons using the identifier ‘W’ and oils for hotter, summer conditions using just a number. The higher the number, the more viscous the oil.

Multigrade SAE gearbox oil grades include two numbers separated by a ‘W’ (75W140, for instance), with the initial number before the ‘W’ indicating performance at 0°C and the number after showing the lube’s performance at 100°C. As with monograde oils, the higher the number, the greater its viscosity.

ISO-grade gear oils

ISO grades are used alongside industrial gear oils. A single number, the higher the figure, the higher the viscosity, with the grade number closely mirroring the mid-point viscosity number in centistokes (cSt) – for instance, ISO 3 equals 3.2 cSt and ISO 220 equals 220 cSt.

Which is better – higher or lower viscosity?

Lower and higher viscosity oils are suited to different applications.

Lower-viscosity gear oils offer better protection and lubrication for high-speed gearboxes that are under relatively low loads thanks to their improved cooling abilities and thinner films which better coat fast-moving components.

Higher-viscosity gear oils offer thick films, better wear resistance and protection from corrosion, making them suited to slower gearboxes that operate under more intense pressures and loads. They also seal components better, affording longer change intervals.

Read our dedicated guide on how to check and change your gear oil and filter.