The processing, production, and transport of heavy crude oils are big challenges for the petroleum industry. Central to this challenge is the fluid viscosity: the key variable responsible for the oil fluidity throughout the entire production process. From the reservoir to delivery conditions, oils undergo large variations in temperature and pressure, which may cause important phase behavior and physicochemical changes, directly affecting the fluid's thermophysical properties. In the case of heavy oils, such a broad change of conditions categorically results in several orders of magnitude viscosity span, including the possible Newtonian to non-Newtonian rheological behavior transitions. It is, therefore, of primary importance that heavy oils be rheologically well-characterized to ensure their production process is successful and viable. The viscosities of heavy and extra-heavy crude oils in extreme conditions (high pressure, high to low temperature, high to low shear rate) are, however, difficult for most service laboratories to fully measure directly. Several lapses may occur when measuring the full range of required conditions using traditional rheometers; for example, for such viscous fluids, the development of laminar-flow structural anomalies (eddies) and magnetic decoupling in the high-pressure cell are common practical problems. In an attempt to pragmatically address these problems, in this work, a methodology that may allow for the rheological characterization of heavy and extra-heavy oils within the full field operational range, but based on limited laboratory measurements, is proposed. The proposed approach does not follow from a simple extrapolation but is rather derived from the concept of control-variable shifting. For achieving this, the superposition principle is applied to shear-temperature and shear-pressure reliable measurements to construct master curves to rheologically characterize the fluid within conditions that may be too severe for direct laboratory measurements. This methodology has been successfully applied to a database of 20 Mexican fluids, going from extra-heavy to light fluids. The rheologies of the samples were originally studied using three different types of equipment: (1) a strain-controlled rheometer (for the measurement of the fluid rheology at ambient pressure and different temperatures), (2) a sliding piston viscometer for high-pressure and low-shear-rate viscosity measurements, and (3) a hybrid rheometer coupled with a pressure cell for the estimation of the fluids rheological behavior under pressure and high shear rate. The rheological behavior of crude oils could then be obtained at conditions as severe as the equipment allowed (up to 1000 bar and, in some cases, shear rate up to 1000 s(-1)). The master curves allowed, however, to extend the rheological characterization of the fluids within conditions that were beyond the laboratory capabilities.