Ferrofluids are stable colloidal dispersions of magnetic nanoparticles in carrier liquids. Their combination of magnetic and fluidic characteristics not only inspires fundamental inquiries into forms and functions of matter but also enables diverse applications ranging from sealants and coolants in mechanical devices to active components in smart materials and soft robots. Spurred by such fundamental and applied interests, a growing need for easy-to-synthesize, high-quality ferrofluid exists. Here, we report the facile synthesis and comprehensive characterization of a silicone oil-based ferrofluid that displays the characteristic surface instability of high-quality ferrofluids and demonstrate its functions in smart interfacial materials and soft robots. Silicone's chemical immiscibility with polar solvents and its biological inertness, when coupled with magnetic responsiveness and fluidic deformability, enable the manipulation of solid particles, gas bubbles, simple and complex liquids, as well as micro-organisms. We envision that the silicone oil-based ferrofluid will find applications in diverse areas, including magnetic digital microfluidics, multifunctional materials, and small-scale robotics.