DOT 3 and DOT 4 brake fluids are chemically similar but have some key differences. DOT 3 brake fluid is the most common type and has a minimum dry boiling point of 205 degrees Celcius. The wet boiling point is 140 C. The dry boiling point is the point where the brake fluid itself boils and the wet boiling point is where the fluid mixed with 3.7 percent water by volume boils. This accounts for moisture in the air getting into the brake fluid in your car. DOT 4 brake fluid has additives that raise the dry boiling point to 230 C and the wet boiling point to 155 C. Since DOT 4 can handle higher temperatures, it’s used in performance cars where the brakes can get hotter in high-performance driving. DOT 4 brake fluid needs to be changed more frequently than DOT 3.