What is Transmission Fluid and What Does it Do?

what is transmission fluid

Transmission fluid is used to lubricate the components of a car’s transmission for optimum performance. In vehicles with automatic transmissions, this fluid also acts as a coolant. There are several types of auto transmission fluids, and the type used in individual cars and trucks depends on the type of transmission inside. Automatic transmissions use regular automatic transmission fluid, as the name suggests. Manual transmission fluid can vary, however, using either plain motor oil, a transmission oil known as heavyweight hypoid gear oil, or automatic transmission fluid. The type of transmission fluid to use in vehicles with standard transmissions can usually be found in the maintenance section of the owner’s manual.

While the primary function of auto transmission fluid is to lubricate the various parts of the transmission, it can serve other functions as well:

  • Clean and protect metal surfaces from wear
  • Condition gaskets
  • Enhance cooling function and reduce high operating temperatures
  • Increase rotational speed and temperature range

Different types of transmission fluid

There are also a myriad of different types of transmission fluid that go beyond merely dividing between automatic and manual transmissions. To achieve the best performance under high temperatures and extend the fluid’s life fully, use the transmission oil or fluid variety recommended by your vehicle’s manufacturer, which is usually found in the owner’s manual:

  • Dexron/Mercon: These varieties, available in different quality grades, are the most commonly used automatic transmission fluids today and contain friction modifiers to best protect the internal surfaces of the transmission.

  • HFM Fluids: Highly Friction Modified (HFM) fluids are much like Dexron and Mercon types, but the friction modifiers in them are even more effective.

  • Synthetic Fluids: These types of fluids often cost more than Dexron or Mercon, but they are designed to better withstand drastic temperature changes and greatly reduce friction, oxidation, and shearing.

  • Type-F: This type of automatic transmission fluid is used almost exclusively in vintage 70s model vehicles and does not contain friction modifiers.

  • Hypoid Gear Oil: Used in certain manual transmissions, this type of transmission oil is highly resistant to extreme pressures and temperatures.

  • Motor Oil: Although motor oil is usually used in a vehicle’s engine, it works in a pinch to lubricate manual transmissions because it has a similar composition and properties to that of gear oil.

Depending on the type of vehicle you own and the length of ownership, you may never have to worry about the type of transmission fluid you use. That’s because it’s not necessary to change it frequently. In fact, some automatic transmissions may never require the fluid to be changed, although most mechanics recommend changing it every 60,000 to 100,000 miles. Manual transmissions require more frequent transmission oil changes, usually every 30,000 to 60,000 miles. If you are in doubt of whether your vehicle needs fresh transmission fluid or oil and what type to use, don’t hesitate to consult with one of our experienced mechanics.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Car trouble?

Our mechanics make house calls.

  • Get a quote & book a service online
  • Save up to 30% compared to shops
  • Repairs, maintenance & diagnostics
  • 12-month, 12,000-mile warranty
GET A QUOTE

Excellent rating

Rating@3x

Need a mechanic?

Our certified mobile mechanics make house calls in over 700 U.S. cities. Fast, free online quotes for your car repair.

Get a quote

Post a question and get free advice from our certified mechanics.


Ask a question