Technologies for reducing the viscosity of heavy crude oil are disclosed. In embodiments the technologies utilize a combination of a processing additive composition (PAC) and hydrodynamic cavitation to produce an oil composition having a viscosity V2, wherein V2 is at least 40% less than a viscosity V1 of untreated heavy crude oil. PACs, systems for reducing the viscosity of heavy crude oil using a combination of a PAC and hydrodynamic cavitation, and methods for reducing the viscosity of heavy crude oil with a combination of a PAC and hydrodynamic cavitation are also disclosed.