Short answer
DOT 4 and DOT 5.1 are both glycol-based and compatible with each other. The difference is in the FMVSS 116 minimums: DOT 4 needs 230 °C dry and 155 °C wet boiling points and can be up to 1,800 mm²/s at minus 40 °C, while DOT 5.1 needs 260 °C dry, 180 °C wet and no more than 900 mm²/s. Use the grade your vehicle specifies.
The specification side by side
Dry boiling point minimum: DOT 4, 230 °C (446 °F); DOT 5.1, 260 °C (500 °F). Wet boiling point minimum: DOT 4, 155 °C (311 °F); DOT 5.1, 180 °C (356 °F). Maximum viscosity at minus 40 °C: DOT 4, 1,800 mm²/s; DOT 5.1, 900 mm²/s.
Both must be colorless to amber, both absorb moisture, and both pass the standard’s compatibility test with other glycol fluids.
Why the minimums are not the whole story
The grade only sets floors. A premium DOT 4 can have a higher printed dry boiling point than some DOT 5.1 products, while still failing to meet DOT 5.1’s cold-viscosity limit. Low-viscosity DOT 4 fluids also narrow the gap on cold flow. Compare the boiling points printed on the containers, and treat the grade as the minimum the vehicle needs.
Choosing between them
Start with the master cylinder cap or owner’s manual. If it says DOT 4, a DOT 4 fluid meets the requirement, and many makers also allow DOT 5.1. If it says DOT 5.1, use DOT 5.1, because DOT 4 does not meet its limits. Neither should go into a system designed for DOT 5 silicone fluid.
Mixing during a top-off
Adding DOT 4 to a DOT 5.1 system will not cause separation, but the blend falls below DOT 5.1 requirements. For a planned grade change, flush the system completely rather than topping off.
Frequently asked questions
Is DOT 5.1 always better than DOT 4?
It meets higher minimums, but the right choice is the grade the vehicle maker specifies. Some systems are developed around a particular fluid.
Does DOT 5.1 absorb water more slowly?
Both are hygroscopic. Rates depend on the formulation and the system, so follow the change interval in the service manual.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].