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.
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.