The reason brake fluid can be Dot 3&4 - Viscosity!

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I was just reviewing FMVSS 116, the US government standard for brake fluid.

Apart from the well known dry and wet boiling point differences between Dot 3, 4 & 5, there is another specification difference: Viscosity at -40 c/f.

The maximums are:

Dot 3 - 1,500 mm2/s
Dot 4 - 1,800 mm2/s
Dot 5 - 900 mm2/s

So technically speaking, a DOT 4 does not automatically meet DOT 3. Whereas a DOT 5.1 meets 3&4 even if not labelled.

Someone living in a colder climate looking to upgrade to a DOT 4, may want to use a DOT 3&4 labelled product to ensure compatibility. Otherwise, it appears there is nothing to worry about. All other specifications, including corrosion resistance are identical.
 
Good observation. But most LV fluids are expensive compared to readily available Dot 4 fluids that will have sufficient Viscosity to meet Dot 3 requirements.
 
Yes, several Dot 4 fluids meet the -40 c/f Viscosity requirement of Dot 3. Castrol also say so on their website.
 
I'm getting to do a brake fluid flush on my mom's Highlander (which specifies DOT 3). Is there any advantage to using a DOT 3 (singularly labeled) fluid over a product labeled DOT 3/DOT 4?
 
^ without having detailed specs, we can't tell for sure, but most likely no. The dot 3&4 brake fluid will have either a higher dry boiling point, a higher wet boiling point or most likely both are higher.

I would say the wet boiling point is the most important spec for daily drivers and I've never seen a dot 3 with a higher wet boiling point than a dot 4. Dry boiling point is different eg Motorcraft dot 3.
 
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