Accurately measuring the rheology of fluid systems under downhole conditions is recommended for the success of well construction. The rheological fingerprints of fluids and their admixtures deployed downhole during wellbore servicing provide a basis for predicting interfacial fluid movements and associated effects on bottomhole circulating pressures. Rheology measurements can also provide a direct indication of any detrimental physical and/or chemical reactions that might occur when the fluids intermix and/or contaminate one another during and after the placement process. A need has existed for some time within the industry for high-pressure/high-temperature (HP/HT) rheology equipment capable of in-situ fluid transfer to change compositions on-the-fly, and easy to clean and maintain considering the abrasive and settable nature of cementitious fluids. This paper discusses an innovative design developed for a fully automated slurry rheometer capable of dosing contaminant fluids in and out of fluid samples to vary composition, mix homogenously in-situ, and measure compatibility between fluid systems at various volumetric compositions—all while maintaining in-situ wellbore test conditions. The rheometer cell is a four-piece design that houses a novel magnetically coupled double helical rotor with cut flights and intermeshing helical blades on the stator to allow mixing and measuring at the same time while overcoming operational issues, such as wall-slip, particle settling, sample coring, and aid in-situ homogenization. A first of its kind, noncontact, zero friction magneto-resistive torsion measurement module with selectable range capabilities and large separation offset (6 to 8 in.) from moving parts is discussed in addition to its usability on both thick pastes as well as thin fluids. A separate design of a high pressure slurry dosing unit that allows for compatibility measurements is discussed. Operating principles, design concepts, engineering development for modularity, calibration data, and slurry test results are presented.