Currently, there are two main types of fluids used in bicycle disc brakes DOT fluid and mineral oil, DOT fluid was traditionally used in car brakes and other bigger vehicles, and even now they are mostly used in them but they are also used in different cycling forms as well like in mountain bikes, road bikes and in other forms of cycling while mineral oil was also introduced in bikes as well for its availability and because it was cheaper
But they are standardized now and using them in any other system like using DOT fluids in brakes that use mineral oil would cause problems and vice versa for the short-term answer as to what will happen if you use DOT fluid in mineral oil brakes or Mineral oil in DOT fluid brakes
Using DOT fluid in brakes made for mineral oils(Shimano, Magura) or Using mineral oil in brakes made for DOT fluid(SRAM, Avid) in both cases won’t work, brakes made to be used with their respect fluid (DOT, mineral oil) have different types of seals and they will destroy the seals and you run the high risk of brake failure
There are many problems with using different types of braking fluid in different brakes like DOT in Shimano brakes and the major problem is the rubber seals materials for both liquids act differently if you try using them with different braking fluids Mineral oil brakes (Shimano, Magura, and others) have NBR seals while brakes which utilize DOT fluid as braking fluid have EPDM seals.
Both these rubber types are different from each other and when used with other types of braking fluids they start to rot and that’s called rubber rotting, the rotting can occur instantly the inner seals swell and start to break or it may take a little time (less than a day) but eventually, the rubber seals will break and the brakes will fail.
What’s the function of seals in bike brakes :
There are different rubber seals present in different bike brakes the seals for DOT fluid are EPDM as we have discussed above and for mineral oils, these seals are made of NBR and they perform a very basic function of preventing the hydraulic brakes liquid from leaking
These seals are present both on the handlebar in the lever and inside the calipers of the brakes as well as right behind the pistons of hydraulic brakes and they prevent the liquid from leaking out as well as prevent water from entering the tubes as well
Using the right seals for the right type of liquid (DOT, Mineral oil) is very important as I have explained above as well.
Effects of different brake fluid on Seals explained :
NBR seals used in bike brakes that use mineral oil are made of nitrile butadiene rubber and DOT fluid will react with them similarly, EPDM rubber seals are made of ethylene propylene diene monomer and they will not work with different types of brake fluids and with different types of rubber seals(O-rings) as well
For example, if you put an o-ring of natural rubber in brakes that use mineral oil or DOT fluids natural rubber will swell and defect in both cases and there is no exception that both of these will act as toxic substances to rubber.
On a general level against human health, mineral oil is considered better as it’s not toxic to humans but DOT fluid can be and that’s why they always advise people to use DOT fluid with gloves and safety DOT fluid can also be toxic to paint and may remove it and its also more toxic to the metal as well.
But It’s imperative to keep in mind that braking fluids are compatible with some rubber types only they are not compatible with natural rubber seals and if one type of rubber is compatible with the mineral oil it may not be compatible with DOT fluid and vice versa.
what can you use as an alternative for DOT or mineral oil :
The other big issue for you now will be what can you use as an alternative if not any DOT fluid or mineral oil there are already tons of alternatives available for example for DOT fluid you can use other DOT fluid types, for example, SRAM and AVID use DOT 5.1 than you can use other DOT fluids such as DOT 3 or DOT 4 both can be used in place of DOT 5.1 but be careful some DOT fluids are different than others as well and I have a complete article on that as well by
Can you use car brake fluid in a mountain bike (precautions to keep in mind)