The effect of Fe₃O₄ nanoparticles and carbon nanotubes (CNTs) on the viscosity of a nanofluid is experimentally investigated from 278 to 313 K by changing the nanoparticle volume fraction. These nanoparticles were put into distilled water with various surfactants, i.e., Colace (docusate sodium), trisodium citrate dihydrate (TSC), polyvinyl pyrrolidone, cetyl trimethylammonium bromide, tetramethylammonium hydroxide (TMAH), acacia senegal (GA), sodium dodecyl benzene sulfonate, sodium dodecyl sulfate (SDS), and sodium laurylsulfonate (SLS). Based on the present measurements, new empirical formulas are proposed for Fe₃O₄–water, CNT–water and Fe₃O₄–CNT–water nanofluids to provide accurate predictions for the nanofluid viscosity. Based on the viscosity testing, stabilities and thermal conductivities of Fe₃O₄–TMAH, Fe₃O₄–Colace, Fe₃O₄–TSC, CNT–SDS, CNT–GA, Fe₃O₄–CNT–SLS, and Fe₃O₄–CNT–TSC nanofluids with a volume concentration of 0.5% are investigated in the present research. Results indicate that better stability, smaller viscosity, and higher thermal conductivity are obtained, when the surfactants TMAH, SDS, and SLS are added into the Fe₃O₄–water, CNT–water, and the Fe₃O₄–CNT–water nanofluid, respectively. The CNT–water and Fe₃O₄–CNT–water nanofluids exhibit a shear-thinning behavior, whereas a linear rheological behavior can be observed by water-based Colace–Fe₃O₄, TMAH–Fe₃O₄, and TSC–Fe₃O₄ nanofluids.