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About gear oils & differential fluids
What gear oil actually does
Gear oil sits between metal teeth that push against each other under very high pressure. It has to stay thick enough under that load to keep the surfaces apart, thin enough when cold to reach every tooth at start-up, and stable enough not to break down when the axle heats up. That is why gear oil is much thicker than engine oil and smells different: it carries extreme-pressure additives that engine oil does not.
Reading the grade on the bottle
A grade like 75W-90 has two halves. The first number with the W is the cold rating: how easily the oil flows in winter, when a lower number means better cold flow. The second number is how thick the oil stays at operating temperature, around 100 °C. A 75W-90 and an 80W-90 behave almost the same when hot; the 75W simply moves better on a cold morning.
The grade alone does not tell you whether the oil suits your gearbox. That is the job of the API rating.
GL-4 or GL-5: the choice that matters most
GL-5 carries more extreme-pressure additive and is made for hypoid differentials, which is what most rear axles use. GL-4 carries less, and many manual gearboxes with brass or bronze synchros are built for it. Putting GL-5 in a gearbox that asks for GL-4 can wear those soft parts over time, which is why some manual boxes need a GL-4 or a dual-rated oil. Your owner's manual settles this in one line, and it is worth checking before you buy.
Limited-slip differentials
If your differential is limited slip, it needs a friction modifier, either already blended into the oil or added separately. Without it, you may hear chatter or shudder from the rear when turning slowly.
How much you need
Most car and light-truck differentials hold somewhere between 1.5 and 3.5 quarts, and gearboxes vary just as widely. The fill level is set by the fill hole, not by a dipstick: with the vehicle level, oil should sit just at the bottom edge of that hole. Our capacity guides list figures for specific vehicles, but the manual for your exact model and axle is always the final word.
Common mistakes
Using engine oil in a gearbox, topping up with a different rating than what is already in there, filling with the vehicle on a slope, and reusing the old fill plug washer are the four we see most often in owner reports. None of them is expensive to avoid.
Popular in this category: 80W-9075W-90GL-4GL-575W-8585W-14080W-14085W-90