Brake fluids are all technically synthetic and are not petroleum based. The Department of Transportation (DOT) class system sets standards and testing procedures only; the ingredients used are up to the manufacturer. If it meets the standards, it will qualify as “brake fluid.” Based on a combination of the properties determined by testing, glycol based brake fluids are labeled DOT 3, 4, or 5.1.
DOT 5.1 brake fluid was once a specialist brake fluid used in race cars and other high performance application. But, recently it has started to show up on the shelves of some retail and internet part sources.
DOT 5.1 is totally different than DOT 5 brake fluid. Think of 5.1 as a glycol based DOT 4 fluid that meets DOT 5 silicone-based fluid standards. The 5.1 fluids are used primarily in vehicles equipped with ABS brake systems.
Unlike glycol based DOT 3,4 and 5.1 fluids, DOT 5 silicone brake fluid will not absorb water from the atmosphere or act like a paint remover. Silicone has very high dry and wet boiling points. It is also more compressible and can absorb more air than a conventional brake fluid.
DOT 5 does have its applications. If you own an extremely rare car that has expensive or original paint, DOT 5 is the fluid for you because it will not eat away the paint if spilled. But, if you use DOT 5, you are almost required to bleed the brakes before you go for a spin. Why? DOT 5 is lighter than water. Any moisture will pool at the lowest point which is typically the calipers or wheel cylinders. Many classic cars have been lost after the owner decided to shake loose the cobwebs before bleeding the brakes.