Newtonian or non-Newtonian fluid from data

A Newtonian fluid obeys $\tau=\mu\,\dfrac{du}{dy}$, so the ratio $\tau/(du/dy)$ is constant and $\tau$ against $du/dy$ is a straight line through the origin.

Apparent viscosity $\mu_a=\tau\big/\dfrac{du}{dy}$ for each reading:

The ratio is not constant: it falls steadily from 0.0178 to 0.0047 Pa·s as the shear rate rises, so the plot of $\tau$ against $du/dy$ is a curve, not a straight line.

A log-log fit of the data gives the power-law (Ostwald-de Waele) form

$$\tau=K\left(\frac{du}{dy}\right)^{n},\qquad n\approx0.74,\quad K\approx0.0196\ \text{Pa·s}^{n}$$

Since $n<1$ and the apparent viscosity decreases with shear rate, the liquid is non-Newtonian, of the pseudoplastic (shear-thinning) type.