Somewhere, on the body of your hydraulic brake lever, the reference “DOT 4 only” or “Mineral Oil” is specified. These two labels refer to the type of brake fluid flowing through the brake hose, responsible for operating the brake pads when you pull the lever.
Mineral oil and brake fluid have two important features: compressibility and boiling point. Compressibility level should remain as low regardless of the temperature, so your brakes work effectively, regardless of the operating conditions. The higher the compressibility level, brake lever travel will increase and your brakes will be less efficient.
As manufacturers design their brakes to be used either with DOT brake fluid or mineral oil, you won’t have to worry what choice you have to do for your bicycle. However, if you reverse the fluids, you can say goodbye to gaskets, hoses and other components of the braking system.
Shimano, Tektro and Magura use mineral oil and it is strictly recommended to use the manufacturer’s mineral oil (not Magura Royal Blood mineral oil in a Shimano system or vice-versa). Avid, Hayes, Hope and Formula use DOT brake fluid.
As for the boiling point, we distinguish the following values: DOT 4 – 230 degrees Celsius, DOT 5.1 – 270 degrees Celsius (a higher value of this indicator shows a higher boiling point. For mineral oil, the boiling point is around 260 degrees. DOT fluid can absorb moisture from the outside, decreasing the boiling point and diminishing braking performance. However, moisture absorption capacity is low and it rarely occurs.
DOT brake fluid is corrosive, and this could be an advantage for mineral oil which does not attack paint or skin. If you happen to pick up a few drops on the brake lever or on the frame and not you don’t quickly remove them with a damp cloth, the fluid will attack the component’s finish. Also, you may feel tingling on the skin.
What you need to remember is that swapping the two types of fluids can damage the brake system and is not recommended. Also, use only DOT 4 and DOT 5.1 brake fluid for Avid, Hayes, Hope and Formula hydraulic braking systems. Do not use DOT 5 brake fluid as it is silicone based and is not compatible with brake gaskets. For Shimano, use Shimano Mineral Oil, for Magura, Royal Blood and for Tektro, Tektro Mineral Oil.