My 2012 Ford Focus manual says to use DOT3 brake fluid, but the cap on the brake fluid reservoir says only use DOT4.
Which one do I use?
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The only difference between the two is the boiling point of the fluid, with DOT4 having a higher range. You could use either and get away with it, but if the cap on the master cylinder states it should be DOT4, that's what I'd go with. Even if DOT3 is all that is required, DOT4 will add a little bit better boil resistance. The two different types can be mixed without issue, but if you mix in DOT3 with DOT4, you should assume the boil temp will be reduced to that of DOT3. There is no longevity difference between the two, either. You should be able to find either type of fluid at any generic parts store.