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Oleic acid coated Mn–Zn ferrite nanoparticles with different compositions have been synthesized by a chemical co-precipitation method to prepare kerosene based ferrofluid. XRD confirms the synthesis of single crystalline phase of Mn–Zn ferrite nanoparticles. The saturation magnetization increases with increase in zinc concentration, reaches maximum at x = 0.4 and decreases for further increase in zinc concentration. The variation in saturation magnetization can be correlated to the modifications in cation distribution as a result of replacement of Mn-ion by Zn-ion thereby modifying the superexchange interaction between the A and B sublattices. The synthesized ferrofluids behaved as Newtonian fluids in the absence of the magnetic field, which naturally reflected the lack of large aggregates. However, under the magnetic field, viscosity increased because of formation of chain-like clusters of nanoparticles showing non-Newtonian behavior of the fluid. The viscosity increases with increase in zinc concentration, reaches maximum at x = 0.4 and decreases for further increase in zinc concentration. The variation in viscosity can be correlated to the modifications in saturation magnetization as a result of substitution of Zn.