If you want a smoother gear shift and transmission, you should use gear oil. Usually, 75W90 gear oil is used for regular uses. On the flip side, 75W140 oil is used in vehicles that have high horsepower and carry loads.
But what is the rationale behind this distinction? And can I use 75W140 instead of 75W90 gear oil?
Can I Use 75W140 Instead of 75W90 Gear Oil?
Before jumping into the main topic, you should know the key difference and features of each gear oil.
75W90 Gear Oil
This 75W90 gear oil has an excellent load-bearing capacity, and that’s why 75W90 gear oil is used in railway gearboxes.
Apart from this, 75W90 gear oil has excellent oxidation stability. As a result, 75W90 gear oil doesn’t have any chemical reaction with oxygen. Besides, as it has high thermal stability, 75W90 gear oil can function properly at high temperatures.
Other features of the 75W90 gear oil are low pour point and high VI or viscosity index.
75W140 Gear Oil
You need smooth gear transmission with gear insulation in trucks or heavy load carriers. Besides, it requires gear oil with a higher viscosity and a superior load-bearing capacity. Luckily, you can have all these by adding 75W140 oil to your heavy-duty vehicle.
If you consider the viscosity index, 75W140 oil has a VI of 170, which is more than the viscosity index of 75W90. However, both the 75W140 and 75W90 have vapor point at 113-degree Fahrenheit.
What does 75W140 Mean in Gear Oils?
Depending on the viscosity, the Society of Automotive Engineers has classified the gear oils into two categories- monograde and multi-grade gear oils.
Monograde gear oils have only one number. For example, 60,70, 90, 140. This number on the gear oil indicates the oil density at a certain temperature. So, the higher the gear oil number, indicates more oil viscosity.
75W140 is the monograde gear oil. The word “W” in the name indicates winter. It means that the gear oil will be flowable at -40 degrees Fahrenheit. But what does 140 mean in 75W140, you may ask.
140 indicates the kinematic viscosity of the gear oil at 212 degrees Fahrenheit.
Key Differences: 75W90 vs 75W140
· Fuel Consumption
If you consider the fuel consumption, 75W90 is the winner. It is because, in higher temperatures, 75W90 has a thinner viscosity grade compared to 75W140. However, if you are a daily driver, you won’t notice much difference in fuel economy by switching from one gear oil to another. So, you can even consider this point as a tie.
· VI Index or Viscosity Grade
As I mentioned earlier, the higher the value after “W,” the thicker the gear oil will be at a high temperature. So, if you have to drive your vehicle in hot conditions, you should consider using the 75W140 oil.
· Load Capacity
75W90 is not suitable for vehicles that are needed to carry heavy loads. If you need to drive your vehicle for hours, where you need to press the accelerator and brake paddle frequently, you should consider using the 75W140 gear oil. With this gear oil, your vehicle gearbox will have less wear and tear. Also, it will help you to increase the longevity of your vehicle.
Verdict on Which Gear Oil to Use
If your vehicle is used for everyday transportation, you should use the 75W90 gear oil. But if you have a truck or pickup, you should consider using the 75W140 gear oil. Apart from the load-bearing capacity, you should consider the weather condition as well. For example, lubricants with lower viscosity are suitable for the gearbox during winter. This is why, during winter, it is advised to use the 75W90 oil in your vehicle’s gearbox. But during summer, you’ll have to start using 75W140 gear oil instead of 75W90. Because in summer, you’ll require gear oil that can handle the excess heat, and only oil with higher viscosity can take such heat.
Compatibility of Vehicles
As I mentioned above, 75W140 is suitable for vehicles with high horsepower. So, if you have a recent Mustang, you should use the 75W140 gear oil in the gearbox.
BMW recommends its users to use 75W140 gear oil in the gearbox. But you can use 75W90 gear oil interchangeably in your BMW car. However, there is one downside of using 75W90 oil in your BMW is that you will have to shorten the oil drain interval.
It is always recommended to use the gear oil that is mentioned in the OEM.
If you’re a Subaru owner, you can use 75W90 gear oil. However, if you are driving your Subaru in hotter conditions, you can switch to the 75W140 gear oil.
Frequently Asked Questions about Gear Oils
In-room temperature, both the gear oil has the same density. However, when it is the operating temperature, the 75W140 is thicker than the 75W90.
For passenger vehicles, you should use 75W90 gear oil. However, in summer or depending on your location’s temperature, you may need to switch to 75W140 gear oil. Consult nearby car mechanics to know more about which gear oil to use in which season.
Yes, both the gear oil has the same viscosity at a lower temperature. It means you can use both the gear oil interchangeably in as low as -40 degrees Fahrenheit.
While picking the right gear oil, you should consider the viscosity, oil type, and additives added to the gear oil.
Conclusion
While adding gear oil into your vehicle, it is always recommended to use the gear oil that is OEM recommended. Otherwise, there is every possibility that your vehicle’s warranty will get void.
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