DOT 5 is one of several North American designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.DOT 5 is a silicone-based brake fluid (contains at least 70% by weight of a diorgano polysiloxane).Unlike polyethylene glycol based fluids, Dot 5 is hydrophobic. An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range.