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Using the Right Fluid for Your Transmission

car transmission

If you are a fleet owner, enthusiast, or just care about your vehicle, it pays to keep a bottle of all the potentially refillable fluids in your garage or boot, just in case. But some of us don’t always do this, perhaps our garage is really full, or we just haven’t got around to keeping a range of refill fluids. Using the right fluid for your transmission is paramount when maintaining the health of your transmission.

Fortunately, most garages and shops have a good alternative, but it’s still imperative to get the right oil for the job.

When looking at fluid charts, it pays to have some background knowledge of what is recommended and which options can be used together so as to make an informed decision.

What do the SAE Codes Mean?

The abbreviation SAE stands for the Society of Automotive Engineers. The SAE applies codes to oil that denote the rated viscosity. For an SAE rating, oils are tested according to international standards, usually, those created by ASTM, American Society of Testing and Materials, an international non-profit standards organization which defines industry accepted tests for classifying physical properties of substances.

For oil, the SAE codes primarily relate to viscosity, that is the thickness or thinness of the oil. An oil with a high viscosity is very thick, closer to grease, an oil with a low viscosity is thin like water.

The ‘W’ in the SAE rating denotes winter, reminding us that the first number is a low-temperature rating, and the high number is a high-temperature rating for the oil.

E.g. for SAE 10W-30, 10 is the winter rating, low-temperature viscosity at zero degrees, and 30 is the high-temperature rating, at 100 degrees Celsius.

Low Viscosity Versus High Viscosity Rating

Where there is a range of oil options for your transmission, the choice will depend on your operating environment. An engine with a lower “W” number will have more fluid oil in cold temperatures.

If ambient temperatures are very high, engine operating temperatures will be higher and you will need a higher viscosity number to prevent excessive thinning. For example, 0W-20 or 5W-30 oils are suitable for cold climates, 15W-40, or 20W-50 are better for hot environments. It might be easy to remember as the lower or higher the temperature, the lower or higher the SAE number selected within the permitted range.

Monograde oils, that is, oil with just a single SAE rating, are no longer used for automotive oil grades, but may still be specified for vintage engines.

What is an API Rating?

An API rating is a two letter code given to oils by the American Petroleum Institute. Wherever an API code is specified, the oil must be the same API code.

API certified oils have the API donut around the SAE number with the two letter code following the words ‘API Service’ e.g. ‘API Service SJ’.

Why is There Such a Big Difference in Types of Transmission Oil?

Different engine transmissions have a multitude of different operating configurations. Some will operate on engine oil, whereas others need a much thicker grade, that is, a higher SAE number.

The mechanics of individual gearboxes and their design operating temperatures will specifically dictate which oil is required. This is why it’s important to consult a transmission fluid chart to determine which oil is suitable for your gearbox.

What Are The Other Types of Transmission Fluids Available?

Some transmissions recommend a manufacturer specific gear oil, designated OEM, for original manufacturers equipment specification. Others may permit a multi-use lubricant like Syncromesh MTF (manual transmission fluid). The permitted fluids are specific to the engine and transmission design.

What is the Difference Between Different Fluid Brands, and Can Brands Be Mixed?

Different brands of oil contain very different additives, which can react with each other to create an adverse effect on lubrication properties. While mixing oils is acceptable, it is vitally important that they have the same SAE and API rating, and that you only use oils approved for your car. It is best to change the oil entirely when considering a new brand.

Getting the Recommended Brand For Your Transmission

If you are not sure about the oil in your transmission, the best step is to talk to a professional. High Gear Transmission has provided a manual transmission fluid guide online for your convenience, but if you have any questions feel free to contact us.

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