Any small molecule fluid is Newtonian. This is clear theoretically, so that people do not even bother to say that CO2 is Newtonian. The non-Newtonian fluids are relatively long polymers, so that the physical interactions have extra degrees of freedom describing how the polymers are tangled up and slide past each other. When there are no polymers, the x,y,z components of the fluid momentum are each locally doing independent diffusion at small scales, just on general principles of physics, any long-range random drift of a conserved quantity is diffusive by the laws of large numbers.
This is generally stated as a form of the central limit theorem--- when you have a conserved quantity moving from point to point at random, the quantity is doing diffusion at long distance scales. Diffusion of momentum is Newtonian viscosity. But of course it helps to have experimental data to make sure you aren't missing something, which akhmeteli provided.