Buy 75W-90 gear oil

75W-90 is the most common grade for modern manual gearboxes and final drives. What decides here is not the viscosity alone but the API class: GL-4 for gearboxes with non-ferrous synchro rings, GL-5 for hypoid axles - mixing the two up costs you the gearbox.

GL-4 or GL-5 - the most important difference

With gear oil the performance class is more dangerous to confuse than with engine oil, because both oils are sold as "75W-90":

  • **API GL-4** is made for manual gearboxes with synchro rings. Its additive content is measured so that it does not attack non-ferrous metals (brass, bronze).
  • **API GL-5** is for hypoid gearing in final drives, with a far higher share of sulphur-bearing extreme-pressure additives. In a manual gearbox with brass synchro rings these additives can attack the rings over time - the result is a gearbox that no longer shifts cleanly when cold.

Some oils are approved for both (GL-4/GL-5). If your manual calls for GL-4, do not fill in a pure GL-5 just because the viscosity matches.

LS additive for limited-slip differentials

Vehicles with a limited-slip differential need an oil with a limited-slip additive (often "LS" in the product name). Without it the diff judders and makes noise; the other way round, an LS oil in a differential without a limited-slip unit does no harm. If in doubt, the manufacturer's figure decides.

What the numbers mean on gear oil

The SAE scale for gear oils is a different one from the scale for engine oils - a 75W-90 is not "thicker" than a 5W-40, the numbers are simply not comparable. Within the gear oil scale the same logic applies: 75W describes cold behaviour (important, because a stiff gear oil makes first and second gear hard to select in the morning), 90 the viscosity at operating temperature.

Alternatives in our range: 80W-90 for older axles and commercial vehicles, 75W-80 for gearboxes that explicitly call for a thinner oil.

Changing and fill quantity

A gear oil change is not in every service schedule - many manufacturers declare the fill a lifetime fill. That holds for normal use; with trailer loads, short trips or hard driving, a change every 100,000 to 150,000 kilometres is a common and sensible precaution.

Fill quantities on car manual gearboxes are usually between 1.5 and 3 litres, on final drives between 0.8 and 2 litres - far less than on the engine, which is why 1-litre containers are the rule here. We take back used oil in the quantity purchased (used oil disposal); gear oil is expressly included.

Frequently asked questions

What is the difference between GL-4 and GL-5?

GL-5 contains far more sulphur-bearing extreme-pressure additives and is intended for hypoid axles; GL-4 is matched to manual gearboxes with brass synchro rings. GL-5 in a GL-4 gearbox can attack the synchro rings - go by the class your manufacturer names, not just by 75W-90.

Do I need LS gear oil?

Only with a limited-slip differential. Without the LS additive the diff judders; the other way round, an LS oil in an open differential is harmless. The vehicle papers or your workshop will tell you whether a limited-slip unit is fitted.

Does gear oil need changing at all?

Many manufacturers provide for a lifetime fill. With trailer use, short trips, high mileage or a stiff shift, a change still makes sense - gear oil ages and picks up metal debris, even where no interval is laid down for it.

Can I use 75W-90 instead of 80W-90?

Often yes, if the API class and any manufacturer approval required both match: 80W-90 is a little stiffer in the cold and practically the same in service. Check the specification all the same, especially on commercial vehicle axles.