A device and method are provided for manipulating petroleum, non-conventional oil and other viscous complex fluids made of hydrocarbons that comprise enforcement of fluid in a multi-stage flow-through hydrodynamic cavitational reactor, subjecting said fluids to a controlled cavitation and continuing the application of such cavitation for a period of time sufficient for obtaining desired changes in physical properties and/or chemical composition and generating the upgraded products. The method includes alteration of chemical bonds, induction of interactions of components, changes in composition, heterogeneity and rheological characteristics in order to facilitate handling, improve yields of distillate fuels and optimize other properties.