The law is not universal in its application. It is applicable to some liquids while other liquids do not obey it.
Newton's Law of Viscosity can be mathematically described as
t = m dv/dy
where t = shear stress
dv/dy is the velocity gradient
Liquids which obey Newton's Law of Viscosity are called Newtonian liquids. Liquids whose response cannot be described by this law are called Non-Newtonian Liquids.