Which is better DOT 4 or DOT 5 brake fluid?
For most drivers, DOT 4 glycol-based brake fluid is the better choice. DOT 5 silicone-based fluid is specialized for certain classic or high-temperature applications and requires careful maintenance to avoid system issues.
Brake fluid is a critical part of a vehicle’s braking system, and choosing the right type depends on your car’s design, climate, and how you use the vehicle. The difference between DOT 4 and DOT 5 boils down to chemistry, moisture handling, and compatibility with seals and other components. Understanding these distinctions helps owners avoid costly mistakes and ensure reliable braking performance.
What are DOT 4 and DOT 5 brake fluids?
Brake fluids are assigned DOT ratings that reflect their chemical makeup and boiling points. DOT 4 is glycol-ether based, while DOT 5 is silicone-based. These fluids have different properties that affect moisture absorption, boiling point under heat, and how they interact with seals and paints in braking systems. A key point is that DOT 5 does not mix with glycol-based fluids like DOT 3, DOT 4, or DOT 5.1, so switching types requires a full system flush.
Chemistry and compatibility
DOT 4 and DOT 5.1 (glycol-based fluids) are compatible with each other and with DOT 3, making them broadly suitable for modern vehicles equipped with ABS and power brakes. DOT 5, being silicone-based, is not compatible with glycol-based fluids and should only be used in systems specified by the manufacturer or in carefully prepared classic/race setups.
Key differences at a glance
The following list highlights the main contrasts between DOT 4 and DOT 5, focusing on how they perform in real-world driving and what that means for maintenance and compatibility.
- Chemistry: DOT 4 is glycol-based; DOT 5 is silicone-based. This affects absorption of moisture, compatibility, and sealing materials.
- Moisture handling: DOT 4 absorbs water over time, lowering the boiling point and necessitating regular flushes; DOT 5 does not mix with water, but any moisture in the system will separate and can promote corrosion if not managed carefully.
- Boiling points: DOT 4 has a lower dry/wet boiling point than DOT 5, but silicone DOT 5 can tolerate higher temperatures when dry; in practice, the need for regular maintenance with glycol fluids often offsets this advantage.
- System compatibility: DOT 4 works with most modern brake systems, including those with ABS; DOT 5 requires special consideration of seals, paints, and compatibility with the vehicle’s components.
- Cost and availability: DOT 4 is widely available and typically cheaper; DOT 5 is more specialized and tends to be more expensive.
In practice, for daily driving and modern vehicles, DOT 4 is usually the sensible choice. DOT 5 is reserved for specific classic cars, certain race-prepped setups, or systems that manufacturers explicitly specify for silicone-based fluid use.
Practical guidance: When to choose which
To help readers decide, consider the following guidelines based on vehicle type, usage, and manufacturer recommendations.
- Follow the vehicle’s manufacturer specification. If the owner’s manual calls for DOT 4 or DOT 4.1, use that exact specification.
- For modern cars with ABS and frequent stop-and-go driving in varying climates, DOT 4 is generally the safer, more practical option due to availability and ease of maintenance.
- For classic or vintage cars that operate in sealed, well-maintained systems and where moisture buildup is less of a concern, some owners opt for DOT 5—but only if the entire system is designed for silicone fluid and compatibility is confirmed by the manufacturer.
- Do not mix DOT 4 (glycol-based) with DOT 5 (silicone-based). If you switch, perform a complete system flush to remove all traces of the old fluid.
- If you are converting from DOT 4 to DOT 5.0, be prepared for additional maintenance, including possible changes to seals and components that must be compatible with silicone fluid.
Concluding guidance: For the vast majority of drivers, DOT 4 remains the recommended choice. DOT 5 should only be used when the vehicle’s design or racing/special-use context explicitly requires it and when the system is prepared for silicone fluid care.
Maintenance and best practices
Regardless of which fluid you choose, regular maintenance is essential. Brake fluid is hygroscopic, meaning it absorbs moisture from the air, which lowers boiling points and can lead to brake fade under heavy use. Typical guidance suggests inspecting and replacing brake fluid every 1–3 years, depending on driving conditions, climate, and manufacturer recommendations. If you suspect moisture contamination, a full system flush is advised rather than attempting to top up with a different DOT rating.
Summary
DOT 4 is the general-purpose, glycol-based brake fluid favored by most modern vehicles for its compatibility, availability, and balanced performance. DOT 5 offers high dry boiling points and moisture resistance in theory but requires careful, system-specific use and is not compatible with glycol-based fluids. For everyday driving, stick with the manufacturer’s specification (usually DOT 3 or DOT 4). Reserve DOT 5 for the limited contexts where a classic or race-oriented system calls for silicone fluid, and ensure a complete flush when switching types. Regular maintenance and correct selection are the keys to reliable braking performance.
Can you mix DOT 5 and 4?
No, you cannot mix DOT 4 and DOT 5 brake fluid because DOT 4 is glycol-based while DOT 5 is silicone-based, and they are not compatible. Mixing them will cause them to not combine, potentially leading to gelling, seal damage, and compromised braking performance. You should never mix DOT 5 with any other DOT brake fluid.
This video demonstrates what happens when you mix DOT 3, 4, and 5 brake fluid: 56ssubtledriverYouTube · Jan 23, 2022
Why you cannot mix DOT 4 and DOT 5
- Different compositions: DOT 4 is a glycol-based fluid, while DOT 5 is made of silicone. They are chemically incompatible, which is why they separate rather than mix.
- Potential for damage: The incompatible fluids can lead to a variety of problems, including seal damage, gelling (creating a thick, viscous substance), and a significant loss of braking power.
- System failure: Mixing these fluids can lead to brake failure, making it extremely dangerous.
What to do if your system needs fluid
- Check your manual: Always consult your vehicle's owner's manual to determine the correct type of brake fluid it requires.
- Flush the system: If your system contains the wrong type of fluid, a complete flush is necessary to remove all the old fluid before refilling it with the correct type.
- Do not top off: Never simply top off a system with a different type of fluid than what is already in it.
What are the disadvantages of DOT 5 brake fluid?
DOT 5 - the silicone-based alternative
While this may seem to be an advantage, DOT 5 also has a higher boiling point and may be less efficient in extreme temperature conditions. In addition, it is not compatible with systems designed for glycol-based brake fluids.
Should I use DOT 4 or 5 brake fluid?
Best practice is to meet the specs on the cap. If you have anti-lock brakes you MUST use Dot 4 (not sure why). But if conventional brakes, you CAN switch to Dot 5. Dot 4 picks up moisture and causes brake fading. Dot 5 has a higher boiling point, doesn't pick up moisture, and won't destroy your paint if you spill it.
Is it okay to use DOT 5 instead of DOT 4?
No, you cannot use DOT 5 instead of DOT 4 because they are incompatible; DOT 5 is silicone-based, while DOT 4 is glycol-based, and mixing them will ruin the brake system. You should only use the brake fluid specified by your vehicle's manufacturer, which is typically DOT 4 for most modern cars. Switching from one to the other is a major conversion that requires a complete flushing and a change of seals to be done correctly.
Why you cannot substitute DOT 5 for DOT 4
- Different chemical base: DOT 5 is silicone-based, while DOT 3, DOT 4, and DOT 5.1 are all glycol-based. These two types of fluids do not mix and will cause a system failure if combined.
- Incompatible materials: The seals and components in a system designed for DOT 4 brake fluid are not compatible with DOT 5 fluid.
- Conversion complexity: Switching from DOT 4 to DOT 5 is not as simple as a fluid change. It requires a complete and thorough flush of the entire system, as even small amounts of residue can cause problems.
What to do instead
- Check your manual: Always check your car's owner's manual for the correct brake fluid type.
- Use the correct fluid: If your manual calls for DOT 4, use DOT 4. If you need to switch from DOT 4 to another compatible fluid, you could consider DOT 5.1, which is glycol-based and often a good upgrade that can be used to replace DOT 4 (and DOT 3).
- Avoid mixing: Never mix any brake fluids unless the container explicitly states that they are compatible (e.g., DOT 3 and DOT 4 can often be mixed, but NEVER mix DOT 5 with any other type).
