[box type="shadow"]Regulations regarding what types of brake fluid can be used in the United States are specified by the Department of Transportation in regulation part 571.116.
This regulation specifies the legal required boiling point of brake fluids. DOT 3 is one of the approved brake fluids that falls underneath this regulation. [/box]
[box type="shadow"]DOT 4
DOT 4 brake fluid has a boiling point of 230 degrees Celsius or 446 degrees Fahrenheit, compared to DOT 3 brake fluid that has a boiling point of 205 C or 401 F. Like DOT 3, DOT 4 is also a polyethylene glycol-based fluid that is 100 percent synthetic. DOT 4 is made to work specifically well with anti-lock brake systems, but it is also compatible with conventional brake fluids. Due to its higher boiling point and compatibility with anti-lock brakes, DOT 4 is more frequently used in vehicles today than DOT 3. There is typically very little price difference between DOT 3 and DOT 4, which has also contributed to the popularity DOT 4 brake fluid in vehicles.[/box]
[box type="shadow"]DOT 5.1:
DOT 5.1 brake fluid is also considered a substitute for both DOT 3 and DOT 4 brake fluids. DOT 5.1 has the highest boiling point rating at 260 C or 500 F. However, due to the higher costs of DOT 5.1, it is not used as frequently in vehicles as DOT 4 brake fluid. DOT 5.1 is used primarily in racing vehicles due to its ability to withstand the highest amounts of heat. DOT 5.1 is compatible with conventional brake fluids and vehicles, which allows it to still be a plausible substitute for DOT 3 brake fluids if desired.[/box]
[box type="shadow"]DOT 5 Warning
DOT 5 should not be used as a replacement brake fluid for DOT 3, as DOT 5 is a silicone-based, rather than polyethylene-based, fluid. DOT 5 is primarily used in vehicles that must withstand high temperatures and is not used in the average vehicle. It is not compatible with anti-lock brake systems and if used as a replacement for DOT 3 brake fluid, it can lead to automotive seal damage and brake failure. Be sure to not confuse DOT 5 brake fluid with DOT 5.1, which is a compatible substitute for DOT 3, when replacing your brake fluid.[/box]