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There are a few different options when it comes to brake fluids;
Fortunately the brake manufacturer has made the decision for you and you can't really change between DOT and mineral oil. But if and when you change or add brake fluid - make sure you use the correct type. Brake fluid type can usually be found on the brake lever.
The main differences between DOT fluid and mineral oil is boiling point as well as compressibility. DOT fluids are also corrosive, so mineral oil might be safer and nicer to handle.
Boiling point means the temperature, at which the fluid starts boiling. When fluid boils, it vapourises and turns into gas. If that happens, you'll probably lose all braking power, as energy from pulling the brake lever is buffered by air bubbles, which you'll be compressing instead of moving pistons in brake calipers using fluid as an intermediate. This heat is produced by heavy and long braking and boiling brake fluid can in worst case lead to total brake failure.
Boiling point for DOT 4 is 230 celsius degrees and for DOT 5.1 270 celsius degrees. For mineral oil the boiling point can vary a bit depending on manufacturer, but on average it is around 260 celcius degrees.
The majority of brake systems in available use DOT fluid, which is also used in motorcycles and cars.
The large brake manufacturers have designed their bike brake systems to operate with DOT or mineral oil as follows;
Shimano - Mineral oil
Magura - Mineral oil
Tektro - Mineral oil
Avid - DOT
Giant - DOT & Mineral oil
Hayes - DOT & Mineral oil
Hope - DOT
Formula - DOT
Quad - DOT
In the graph below you can see some boiling points of mineral oils from different manufacturers.
Mineral oil can be stored as long as you want, even if the can is opened. DOT brake fluid will ultimately deteriorate once the bottle is opened.
DOT brake fluid is relatively cheap compared to mineral oil. One liter of Shimano mineral oil costs around 25 euros, whereas DOT 4 can be bought at 10 euros per litre. Of course, both very much depending on the whole supply chain. If you buy DOT fluid in 12 ounce bottles from the local bike mechanic, the price will probably be something else. Prices for reference are shown below.
DOT brake fluid can absorb water, mineral oil can't. This means that if water gets into the system, it will cause corrosion in mineral oil based system, but not for DOT based. Then again it will lower the boiling point of DOT based braking system.
Spilling mineral oil won't destroy the paint on your bike, DOT fluid will. Needless to say it's a bit more unhealthy to get DOT brake fluid on your skin than mineral oil.
The whole brake system is built to be operated either with DOT brake fluid or with mineral oil. If you mix those up, you will destroy your gaskets and hoses. DOT fluids 4 and 5.1 can be mixed with each other. Note that there is also DOT 5 which is a silicone based brake fluid and can not be mixed with other DOT fluids.