DOT 3 is the most common and has been around for ever. Many domestic US vehicles use DOT 3 along with a wide range of imports.
DOT 4 is used by European manufactures for the most part but you are seeing it more and more in other places. DOT 4 primarily has a higher boiling point than DOT 3 and has some additives to help reduce changes in the fluid when moisture is absorbed over time. There are variations of DOT 4 you will see DOT 4 Plus, DOT 4 Low Viscosity and DOT 4 racing. In general you do want to use the type the vehicle indicates.
DOT 5 is a silicon based with a very high boiling point (well above DOT 3 and DOT 4. It is designed to not absorb water, it gets foamy with air bubbles in it and is often challenging to bleed out, it is also not intended for use in ABS system. DOT 5 is generally not found on street cars, though it can be, but is often used in show cars and other vehicles where there is concern of the finish since it tends not to damage paint like DOT3 and DOT4 can. The very high boiling point however makes it more useful in high brake use applications.
DOT 5.1 is chemically similar to DOT3 and DOT4 with a boiling point around that of DOT4.
Now when you use the “wrong fluid” While it is generally not recommended to intermix fluid types, DOT3, DOT4 and DOT5.1 are technically inter-mixable. DOT3 is the cheapest with DOT4 being about 2x as expensive and DOT5.1 being over 10x as expensive. DOT 5 should never be mixed with any of the other fluids, they are chemically not the same and you will end up with issues.
If you have a vehicle designed to use DOT3 and put DOT4 or DOT 5.1 into it then there should really be no adverse effects, though it is not advised that you mix them. With a vehicle designed for DOT4 if you again should have no adverse effects, however with the various types of DOT4 it is possible you could have some long term issues if you leave the fluid in there. If you mix DOT5 with any of the others you will likely notice braking issues, often a soft petal and difficulty bleeding the brakes.
What should you do? Well if you do honestly mix then then you should get your brake system flushed and bleed, refilled with the correct fluid. If you realize the mistake and only added to what is in the reservoir before you drive the vehicle or bleed the brakes any distance you likely can just use something to carefully suck all the fluid from the reservoir and then replace it with the correct type, unless you are driving or bleeding and depressing the petal there is no real way for fluid to get into the lines.
If you mix DOT3, DOT4 or DOT5.1 the world should not come to an end if your drive some and likely not if you don’t do anything, they are technically interchangeable. However if you mix DOT5 with any of them you will have braking issues and need to get the system flushed ASAP. It is not likely to damage the brake system in the short term, but it could result in brake system issues and inability to stop like you want.
Post time: Apr-14-2023
