The Consequences Of Putting Motor Oil In An Automatic Transmission


Motor oil offers plenty of crucial benefits when it circulates throughout your engine. Not only does it lubricate moving parts, but it also cleans the engine internally and prevents it from overheating. But unfortunately, that same oil is incredibly harmful when poured into an automatic transmission system.

Pouring motor oil into an automatic transmission will lead to several negative consequences. Firstly, you’ll see your car flash its warning lights to indicate a problem. Then, you’ll sense a burning smell and hear loud sounds like grinding, both coming from the transmission. Finally, in a worst-case scenario, the motor oil could cause the transmission to stop working entirely due to internal damage.

Putting the wrong fluids into your car’s various systems is terrible news. So, read this guide to understand what happens when you put motor oil in an auto transmission and discover what you can do to minimize or fix the problem.

What Are Motor Oil And Automatic Transmission Fluid?

Before exploring the consequences of putting motor oil in an automatic transmission, you must understand what those fluids are and how they differ.

Firstly, let’s talk about motor oil.

As the name suggests, motor oil is designed to work inside your engine and nowhere else. It’s a lubricant that keeps the engine’s components moving smoothly while preventing overheating.

The substances and any additives inside motor oil make it suitable for working in an environment where combustion occurs. Because of that, engine oil can still deliver its benefits despite catching contaminants from the combustion process.

While the fluid you pour into your engine is a lubricant, the one you use for your automatic transmission is a hydraulic fluid. That means it’s designed to move hydraulic components in the transmission system, where no combustion occurs.

The bottom line is straightforward: motor oil is designed to perform a very different function, so it doesn’t belong to the automatic transmission. On top of that, automatic transmission fluid doesn’t belong inside an engine either.

As such, pouring motor oil into the transmission will not only prevent it from functioning correctly, but it’ll also lead to several negative consequences. You’ll learn about those consequences in detail below.

Read: Honda CRV Automatic Transmission Problems

What Happens If You Put Motor Oil In An Automatic Transmission?

Whether you put motor oil in your automatic transmission deliberately or by mistake, it can cause a handful of problems for your vehicle.

Here’s what will happen if you put motor oil in an automatic transmission:

1. Lights And Sounds

The first noticeable side effect of putting motor oil into an automatic transmission system is that you’ll see warning lights and hear loud noises from under the hood.

The Check Engine light is the most common one that’ll be triggered first. Even though it’s not specific to the transmission system, that will be your first hint of a problem that requires urgent attention.

Besides that, you’ll also hear noises like grinding and squealing coming from the transmission itself. As you read earlier, motor oil doesn’t provide the same properties necessary to keep the transmission functioning.

So, the loud noises you’ll hear come from transmission gears and other components struggling to move efficiently.

Read: Why Automatic Transmission Stuck in Gear?

2. Problems Shifting Gears

The motor oil will also cause performance issues. For instance, you’ll notice the system struggles to switch gears while driving.

As the everyday driver of that vehicle, you’ll have a rough sense of when the automatic transmission will shift gears. However, when you fill it with motor oil, you’ll find that the shifting is delayed or might even happen too soon.

That problem with shifting gears will make your driving experience rough, undermining the comfort of you and your passengers.

Read: Automatic Transmission Repair Cost

3. Gear Slipping

Aside from problems shifting gears, your automatic transmission might also suffer from gear slipping. 

Gear slipping is a condition where the automatic transmission struggles to stay in the gear it’s using. As a result, the transmission will slip back into the previous gear. Overall, you’ll feel that the transmission responds far too slow compared to its typical performance.

In more severe cases, the gears might even shift so far back that the car goes into Neutral even though you haven’t even touched the gear.

Read: Automatic Transmission Warning Light. How To Troubleshoot?

4. Overheating

Motor oil can’t circulate throughout the transmission system in the same way as transmission fluid. So, not only will it fail to lubricate the transmission’s moving parts, but it won’t absorb excess heat from any moving parts.

As a result, heat gets trapped and concentrated in certain system parts, leading to overheating.

The consequence of overheating also creates another noticeable symptom: you’ll sense a burning smell while driving. When you trace the source of that smell, you’ll discover that it comes directly from your vehicle’s automatic transmission.

5. Total Transmission Failure

As you can see, there are plenty of consequences when you put motor oil into an automatic transmission. However, if you continue to drive despite noticing the problems mentioned above, the problem will quickly become more severe.

In the worst-case scenario, your entire automatic transmission will fail completely. In addition, the components inside will suffer too much damage to function at all, rendering your transmission completely destroyed.

Not only will that leave you with expensive repairs and part replacements, but it also means that your vehicle is immobilized and can’t be driven.

Read: Automatic Transmission Have Grinding Noise When Put in Gear. How To Fix?

What Should You Do If You Put Motor Oil In A Transmission?

Suppose you just finished pouring motor oil into your automatic transmission. In that case, you can prevent all of the consequences detailed above as long as you keep the engine off.

If you were to turn the engine on, the transmission components would begin working, and the motor oil would circulate as a result. That will quickly lead to all the problems you’ve read about earlier.

However, you can still protect the automatic transmission if the engine stays off. Here’s how:

  • Firstly, you must drain all the motor oil out of the automatic transmission system. You can do it yourself if you have the knowledge, or get a mechanic to do it at your location.
  • Next, you must flush the entire transmission system clean. This process will likely take some time because you must ensure the transmission is free from any traces of motor oil.
  • Then, top up the correct type of automatic transmission fluid to the proper levels.
  • Lastly, test the transmission system and check for any signs of leaks or other damage. That will complete the process.

The most critical thing to remember is that you must never start the engine if you’ve accidentally poured motor oil into your automatic transmission. That way, the problem remains somewhat contained until you can drain the oil, flush the system, and top it up with the correct fluid.

Final Thoughts

Motor vehicles rely on many different fluids to function correctly. However, those fluids should never be mixed, especially motor oil and automatic transmission fluid.

Putting motor oil into an automatic transmission will have plenty of negative consequences. As the first sign, you’ll see warning lights illuminate on your instrument panel.

Then, you’ll hear grinding noises and notice burning smells coming from the transmission. Of course, the transmission will also suffer performance issues, struggling to switch gears and often slipping back to the previous ones. Lastly, the presence of motor oil in the transmission could cause it to fail completely and immobilize your car entirely.

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