Gear oils and automatic transmission fluids: what to know before a change

Engine oil gets changed regularly and everyone remembers it. The lubricant in the gearbox and the differential is often forgotten, even though it works under very high pressure between the gears. This guide explains what gear oils and automatic transmission fluids are, how to read their specifications and when they should be changed.

Gear oil is not engine oil

Gear oil is designed to protect gear teeth, bearings and synchronisers. It contains extreme pressure (EP) additives that form a protective layer where gear surfaces press hard against each other.

Gear oils have their own SAE viscosity scale, with codes such as 75W-80, 75W-90, 80W-90 or 85W-140. This scale is different from the engine oil scale, so the numbers cannot be compared directly.

API GL-4 and API GL-5

The most important information on a gear oil is its API performance category:

• API GL-4: for manual gearboxes and differentials with moderate demands, where the manufacturer calls for this level.
• API GL-5: for hypoid differentials and applications with very high pressures and shock loads, as is often the case in commercial vehicle differentials.

GL-5 is not simply a "better GL-4". Some gearboxes are designed for GL-4 and their manufacturer does not allow GL-5. That is why the choice is always made on the specification listed in the service book.

Examples from the PARNALUBE range

In the gear oils category you will find, among others:

• PARNALUBE GP SAE 80W-90, for differentials and gearboxes where the manufacturer requires API GL-4.
• PARNALUBE ESX SAE 75W-90, a synthetic API GL-5 gear oil for modern gearboxes and hypoid differentials.
• PARNALUBE EP SAE 80W-90 and SAE 85W-140, API GL-5 for demanding gearboxes and hypoid differentials.
• PARNALUBE FX SAE 75W-80, for modern gearboxes, designed for smooth gear changes.
• GEAR OIL SAE 140 and 90, a monograde API GL-1 gear oil.

Limited slip differentials

Limited slip (LS) differentials need a gear oil with special friction additives so that their clutches work smoothly and quietly. If the vehicle manual calls for an LS lubricant, choose a product that states it explicitly, such as PARNALUBE EP SAE 80W-90 LS, which is also intended for racing cars.

Automatic transmission fluids (ATF)

Automatic gearboxes do not use gear oil but a dedicated fluid, ATF (Automatic Transmission Fluid). ATF lubricates, but it also transmits power in the torque converter, controls clutch engagement and carries away heat. That is why its friction properties matter as much as its viscosity.

ATF specifications are mainly set by manufacturers, the best known being General Motors Dexron and Ford Mercon:

• ATF DEXRON VI is a low viscosity synthetic fluid for modern automatic gearboxes requiring DEXRON VI, also suitable for transfer cases and power steering where specified.
• ATF SUPER covers applications requiring DEXRON II-D and III, as well as specifications such as Ford Mercon, Allison C-4, MB 236.1 and ZF TE-ML 09/11/14, for passenger cars and trucks.

Many modern automatic gearboxes (for example dual clutch or CVT units) require very specific fluids. Before any change, check the exact manufacturer specification.

When to change

There is no single interval that applies to every vehicle. Follow the manufacturer's service schedule. Some gearboxes are described as "filled for life", yet under severe conditions such as towing, constant city driving, high temperatures or racing, many manufacturers call for an earlier check or change.

Signs a workshop should look at:

• hard or noisy gear changes,
• a humming noise from the differential,
• lubricant leaks under the vehicle,
• in automatic gearboxes, delayed engagement or fluid with a burnt smell.

Commercial and agricultural vehicles

In trucks, buses and construction machinery, gearboxes and differentials work under heavy loads for many hours. For tractors there is PARNATRAN, a special lubricant for transmission and hydraulic systems and immersed brakes. See also the whole lubricants for commercial vehicles category.

Need help?

Send us the vehicle or machine, the gearbox type and the manufacturer specification, and we will suggest the right lubricant. Contact us or see all our products.