No, you can’t!!! Brake fluid breaks down into two basic types. Glycol based fluids and Silicone based fluids. DOT 3, Dot 4 and DOT 5.1 are all Glycol based fluids and DOT 5 is Silicone based. The most important thing to remember here is that they CANNOT BE MIXED! If you put a Silicone based fluid into a Glycol based system you will have serious problems. It turns into a sludge like an oily salad dressing and is useless. On top of that it causes huge problems with the seals and rubber components in the system. We have seen the reservoir seals from a DOT 4 system become twice the size when subjected to DOT 5 fluid! It also causes big problems with caliper seals and your brake system will need a lot repair work if you use the wrong fluid.
You need to use the fluid specified by the vehicle manufacturer. Most manufacturers now use DOT 4 Glycol based fluids. There are a number of reasons for this including its user friendly nature. As the fluid gradually takes in more water it starts to change colour. From clear, to yellowy brown or golden, then light brown and finally dark brown. The darker the colour the ‘wetter’ the fluid, or the higher the water content. This is an early warning system that is very useful and it is telling you to change the fluid. In general we recommend the fluid is changed every year and more often in racing conditions.