The rheological behavior of various complex fluids was explored in this experimental study. Nanofluids were obtained by mixing nanoparticles with various solvents. The solvents consisted of metal salt eutectics that melt at high temperatures (exceeding 200 °C) depending on the composition of the metal salts in the eutectics. The rheological behaviors of these high temperature solvents were measured as a function of temperature before and after mixing with different types of nanoparticles (chemical composition, size, shape and concentration). These nanofluids exhibited non-Newtonian behavior (shear thinning behavior) while some of the other nanofluids were surprisingly found to have Newtonian behavior. It was observed that high aspect ratio nanoparticles (e.g., stick shaped carbon nanotubes) were more likely to cause shear thinning behavior of the resulting nanofluids.