Not to be confused with DOT 5.1, a polyethylene glycol based fluid.

DOT 5 is one of several designations of brake fluid denoting a particular mixture of chemicals imparting specified ranges of boiling point.

DOT 5 is a silicone based brake fluid. It is not compatible with any polyethylene glycol based fluids.

Unlike polyethylene glycol based fluids it is not hygroscopic, however this means that instead of being dissolved, any water in the system will eventually find its way to the lowest point in the brake line, corroding it, and potentially boiling and causing brake failure. Because of this, and other reasons (such as compatibility, cost, and lack of advantage over regular fluids) it is no longer used very much.