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Transmission Fluid vs Oil: 3 Key Differences

October 15, 2021

Transmission fluid and oil, although similar in composition, serve very different purposes.

But what are these lubricants used for? 
And how do you know which lubricant to use?

This article will talk about the physical differences between transmission fluid and oil and what happens if you accidentally add engine oil to the transmission. We’ll also guide you through the process of checking your transmission fluid level.

This Article Contains:

Let’s begin! 

Transmission Fluid Vs Oil: What’s The Difference?

Transmission fluid and engine oil are both oils with comparable viscosity ranges. They share similar ingredients for their base oil (usually refined crude oil) and additives. 

The primary difference is that transmission oil is a type of hydraulic oil used in your car’s transmission, while motor oil is meant for engine lubrication. 

There are also many other differences when it comes to these two fluids. 
Let’s take a closer look. 

1. Transmission Fluid Vs Oil: Appearances   

While both transmission fluid and motor oil are oils, they look very different from each other.

Engine Oil Appearance 

Engine oil has the following characteristics:

Transmission Fluid Appearance

Transmission fluid has the following characteristics:

2. Transmission Fluid Vs Oil: Lifespans

Transmission fluid and engine oil are affected differently by mileage and time.

Engine Oil Mileage 

Engine oil reduces effectiveness with time and mileage. Depending on the type of motor oil, you’ll have to replace it every 3000-6000 miles. If you leave it too long, it’ll lose its properties, which can damage your engine.

Transmission Fluid Mileage

In contrast to engine oil, transmission fluid doesn’t need to be changed as often. Manual transmission fluid may require changing between 30,000-60,000 miles, and automatic transmission fluid lasts even longer. These are usually changed around 60,000-10,0000 miles.

It’s not normal for transmission fluid levels to drop in a short time, so if you often notice low transmission fluid levels, get it checked for a transmission fluid leak. 

3. Transmission Fluid vs Oil: Applications 

Both engine oil and transmission fluid serve as a lubricant, but their applications vary in different components of the car. Engine oil is primarily concerned with the internal combustion engine, while transmission fluid is a type of hydraulic fluid that focuses on the steering system. 

Let’s dive a little deeper:

Engine Oil Applications

Engine oil (motor oil) is a type of automotive lubricant available in three varieties — conventional, synthetic, and semi-synthetic motor oil. All three contain base oil and natural or synthetic additives for better performance and fuel economy.

Engine oil is used as an engine lubricant and for keeping it clean. It helps enhance the overall performance of the engine. It is also used for:

Now let’s see what transmission fluid does.

Transmission Fluid Applications

Transmission fluid (a type of hydraulic fluid) is typically available in two varieties — Manual transmission fluid (MTF fluid) and Automatic transmission fluid (ATF fluid).

Due to the addition of additives like friction modifiers and coolant improvers, transmission oil (both MTF fluid and ATF fluid) supports the smooth operation of hydraulic parts. 

It also helps with the following: 

Note: Transmission fluid and hydraulic oil are not the same. Transmission fluid is a kind of hydraulic fluid that supplies power from the engine to the transmission. But there are several other types of hydraulic oil lubricants, including CVT fluid, brake fluid, power steering fluid, etc. 

For example, brake fluid transmits power within the brake system, whereas power steering fluid maintains contact between the steering wheel and front tires. All of these work for a hydraulic system, but they aren’t interchangeable. 

Keeping these applications in mind, what happens if you accidentally use engine oil in the transmission system? 

What Happens If I Use Engine Oil In The Transmission System?

There should be little to no damage in most cases if you accidentally add engine oil to your transmission. Just perform a fluid flush and drain the oil as soon as you can.

However, if you add too much oil to your automatic transmission system and let it remain, you may notice the following symptoms: 

Note: Some manual gearbox units in older vehicles require the addition of engine oil over gear oil. In these cases, you should consult your car’s manual to determine the right gear oil for you.

Having said that, how do you check your transmission fluid? 

How Do I Check The Transmission Fluid Level

Most modern cars don’t come with a transmission dipstick and therefore need professional service maintenance. Your car’s user manual should inform you about the proper procedure.  

However, if your car does have a transmission dipstick, here’s how you can check your vehicle’s transmission fluid level: 

  1. Park your car on a leveled surface. Be cautious of any hot engine components.
  1. The vehicle manual will guide you to check your transmission while the engine is on or off.
  1. Identify the transmission fluid dipstick. Carefully pull it out and wipe it off with a clean cloth. 
  1. Reinsert the dipstick into the transmission fluid. Remove the transmission dipstick again to check the fluid level.
  1. The transmission fluid level should be somewhere between the L and H marks on the transmission fluid dipstick. Low transmission fluid can be an indication of a fluid leak in your transmission. In this case, refill with the appropriate amount.
  1. Get the fluid leak fixed and reinsert the transmission fluid dipstick. 

Note: This procedure applies to both automatic and manual transmission systems.  

You may also want to check your transmission for any signs of old fluid; these include whining sounds, difficulty in changing gears, and thumps between gears. In this case, you might want to get a transmission fluid flush or simply drain the old fluid and get a fluid change. 

What if there’s a transmission fluid leak?

Leaking fluid can be detrimental to your engine. A transmission fluid leak can be due to problems with the transmission oil pan and lead to low transmission fluid levels. If you neglect it too long, it can cause severe engine damage and reduced performance.

Get any leaking fluid fixed as soon as you can, and perform a regular transmission fluid flush to get rid of the old fluid and avoid any problems. Similarly, ensure that you get frequent fluid changes.

If you’re wondering how to change engine oil instead, find out with the help of this guide

Closing Thoughts

Although they may seem similar, engine oil and transmission oil serve very different purposes. Engine oil helps enhance the engine’s fuel efficiency and overall performance while transmission fluid transfers power to the hydraulic parts and vehicle’s transmission. 

However, both these fluids function best when they are regularly maintained
Engine oil needs routine oil changes, while gear oil (MTF and ATF fluid) should be serviced depending on the car’s recommended schedule.

And when you need ASE-certified mechanics for your car maintenance needs, RepairSmith is your best option!

RepairSmith is a mobile car repair and maintenance service that offers online booking and upfront pricing. They can help you with any transmission repair, transmission fluid leak inspection, and more. 

Fill out these forms to get a cost estimate for a transmission fluid change and an engine oil change.