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The world continues experiencing a steady decrease in the availability of conventional light or even medium crude oils, and hence, refineries inevitably face the challenge of processing increasing amounts of (extra) heavy crude oils while preserving their economic targets. This requires changes/improvements in existing refining technologies for converting heavy crude oil, which may be conveniently preceded by an improvement in its flowing properties in the underground reservoir itself. In this context, this research focuses on the use of liquid catalysts containing Cu, Fe, Ni, or Zr for upgrading in situ a heavy crude oil stream (12.6 °API) in a bench-scale batch-stirred reactor operated at 380 °C and 10.8 MPa for 1 h, in the presence of high-purity hydrogen. Upon reaction, the heavy crude oil properties improve while catalysts undergo in situ transformations identifying from the analysis of residual solid recovered from the bottom of the reactor, the formation of some metallic sulfides and oxides (by X-ray diffraction (XRD)), which exhibit Bronsted and/or Lewistype acidity (via Fourier transform infrared (FT-IR) spectroscopy). While converting ca. 43% of the heavy crude oil residue fraction (>524 °C), the Zr-containing catalyst, which exhibits relatively strong Bronsted and Lewis acid sites thus increasing the cracking rate, decreases the kinematic viscosity (1444−24 cSt, at 37.5 °C) as well as the amount of asphaltenes (28−13 wt %) and sulfur (5.14− 2.8 wt %), and increases the content of light and middle distillates (18.9−34.1 wt %) along with the API gravity (12.6−21°). The results indicate that the use of liquid catalyst appears as a promising option on the way to development of an "inside-reservoir technology" for heavy crude oil processing.