Locally beneficiated clays from Pindiga Formation in Northeastern Nigeria have been found to attain standard rheological properties required for Oil and Gas Well drilling after addition of some percentages of poly anionic cellulose. But these properties were measured under ambient temperature (32 °C). There is the need therefore to ascertain the performance of these beneficiated clays at field/down-hole conditions (high temperature and pressure). In this study, three of these clays labeled GHZ, PNG and SGN were chosen because of their high rheological performance among others as already reported in our earlier work, formulated and subjected to different temperature (ambient (32 °C), 40 °C, 50 °C, 60 °C, 70 °C and 80 °C) together with API grade bentonite. At each temperature, their rheological properties were computed to ascertain their performance under these conditions. Physical and other properties (Loss On Ignition, Specific Gravity, Particle Size Distribution, Textural Classification and Formulated CEC) of the sample were also determined and compared with API grade sample. The results obtained indicated that formulation using samples beneficiated with 12% Na₂CO₃ had viscosity values (cP) that ranges from of 2.0–7.9 at temperature ranging from 30 °C to 80 °C with sample PNG recording the highest values of 7.9 cP at 50 °C. For the API grade sample, these values (cP) range from 4.5–16 with highest value recorded at 70 °C. Viscosity results for formulation using same beneficiated sample but with addition of 0.8 and 1.5 g poly anionic cellulose indicated that formulation with 0.8 g poly anionic cellulose had the most impressive improvement among other formulations. The results recorded ranged from 1.0–44 cP with sample PNG recording the highest viscosity of 44 cP at 70 °C. When the Apparent Viscosity (AV), Plastic Viscosity (PV) and Yield Points (YP) of the samples were computed, the results also indicates that sample PNG had the highest values for these parameters compared with the API grade. Other parameters (Loss On Ignition, Specific Gravity, Particle Size Distribution, Textural Classification and Formulated Cation Exchange Capacity) determined had values/behaviors comparable with standard specifications. Thermal stability results indicated that as it is with viscosities of the different formulations at ambient temperature, sample PNG could possibly give a better result (behavior at the measurement conditions) compared with the others even at down-hole conditions and can favorably compete with even the API grade. But at temperatures greater than seventy degrees (70 °C), the performance of both the API grade and the locally beneficiated and formulated drilling fluids began todeteriorate (fail) sharply.