What's the Difference Between DOT 3 and DOT 4 Brake Fluid?

Have you ever wondered which kind of brake fluid your vehicle suits? DOT 3 or DOT 4? Which one will better keep you safe in an emergency?

Imagine driving on the highway, and something unexpected happens. You hit the brakes—but they don’t respond. That’s a terrifying situation, and definitely not one you want to find yourself in. Every vehicle relies on brake fluid—it’s essential for the braking system to work properly. Without it, your safety could be at risk.

Many people understand that brake fluid is important, but they’re not clear on why it matters. If you are or will be a car owner and this is exactly what you are confused about, hope this blog will give you some helpful tips.

DOT stands for "Department of Transportation." It sets the standards for various brake fluids based on their chemical composition and performance. DOT 3 and DOT 4 are the most common types. There are also DOT 5 and DOT 5.1. Each type has different boiling points, moisture absorption rates, and compatibility with braking systems.

DOT 3 brake fluid uses glycol ether as a solvent. It works for all brake systems and driving conditions. DOT 4 brake fluid also uses glycol ether and borate ester compounds. These compounds make it perform better than DOT 3.

Interestingly, there is no unified standard for brake fluid. Different manufacturers may use different formulas. The characteristics may vary, but they all work in the same way. It is stored inside the master cylinder. When you press the brake pedal, brake fluid flows into the caliper. This forces the brake pads against the rotor, creating friction to slow the car down. The harder you press, the shorter the stopping time.

If the fluid doesn’t match your vehicle’s requirements, it can affect brake performance. For example, if the fluid has a lower boiling point than the car needs, this can make it overheat and cause brake fade, making it harder to stop safely. It can also damage brake system parts like seals and rubber, leading to more severe problems. Using the wrong fluid can also cause chemical reactions that make the fluid less effective. This can lead to poor braking and safety risks.

You may wonder what's the difference between DOT 3 and DOT 4 and have difficulty choosing the fluid? Check the table below. The comparison shows the key characteristics of each fluid type. Understanding the differences between different brake fluids is important for ensuring vehicle safety and performance.

DOT 3 has a lower dry/wet boiling point than DOT 4. It absorbs water from the air slowly, so it needs to be changed less often. It works best for older or lighter vehicles. DOT 4 works better in higher temperatures and under heavier loads. It has a higher boiling point, but it absorbs moisture faster. This means it needs to be changed more frequently. To sum up, the primary difference between the two is their heat and water resistance.

If you just need a car for daily commuting, DOT 3 brake fluid will be enough. It is made for lighter, everyday driving. DOT 3 gives good performance in normal conditions. It is used in most standard vehicles and is cheap.

If your car is used for heavier tasks, like hauling cargo or driving in hilly areas, use brake fluid like DOT 4 brake fluid would be a better choice. DOT 4 has a higher boiling point, which helps it resist heat. This makes it better for cars that do a lot of braking or work in tough conditions, like driving uphill or towing. DOT 4 is also better for high-performance cars. It helps keep brakes working well and prevents brake fade in harder driving situations.

1. Can you mix DOT 3 and DOT 4?

YES, but better not to. If you've used DOT 3 before and want to switch to DOT 4, or you accidentally put DOT 3 and DOT 4 together, don't worry—your car will still work fine. Usually, DOT 3 is mixable with DOT 3, DOT 4, and DOT 5.1 non-silicone base products. However, don't mix DOT 5 with either of them. DOT 5 has a very different composition and is not compatible with them.

2. When to change the brake fluid?

In common situations:

DOT 3 needs to be changed every two to three years or 30,000 to 40,000 miles for daily driving.

DOT 4 should be changed every 20,000 to 30,000 miles, as it absorbs moisture 20% faster than DOT 3.

However, their durability also depends on the car's usage. If the vehicle is frequently in a damp environment, the replacement frequency should be increased.