The apparent viscosity of the oil–solid interface in steel rolling oily sludge (SROS) is governed by the amount, identity, and size of solid particles. To decouple these factors, oleic acid was used as a model oil phase, and Fe, Fe₃O₄, and Fe₂O₃ were selected as representative iron-bearing minerals. The effects of mineral type, Fe₃O₄ volume fraction, and particle size on the rheological behavior of oleic acid–mineral suspensions were systematically investigated using a rotational rheometer. Pure oleic acid behaved as a Newtonian fluid (24.96 mPa·s), and all suspensions exhibited near-Newtonian behavior with flow indices close to unity (0.959–1.048) and small yield stresses; no shear thickening was observed. At a matched loading of 4 vol%, the apparent viscosity followed the order Fe₃O₄ > Fe > Fe₂O₃. Notably, the Fe₂O₃ system exhibited a viscosity almost identical to that of neat oleic acid, which indicates that the reactivity of the mineral surface towards the carboxyl group of oleic acid, and the resulting aggregation state, dictate the particle–oil coupling, rather than the degree of oxidation. Furthermore, the apparent viscosity increased monotonically with Fe₃O₄ volume fraction (2–6 vol%) and decreased with increasing particle size (Dv(50) = 13.3–29.2 μm). Interestingly, the yield stress was highest for the coarsest powder, revealing that the static network strength and steady-flow viscosity are controlled by different microstructural features. These findings provide a quantitative basis for interpreting the oil–solid contribution to the viscosity of iron-bearing sludges and for developing targeted viscosity-reduction strategies.