The concentrated solar thermal systems, operating in the medium temperature range of 373–573 K, will be extremely useful for several industrial processes. However, the need for an in-depth understanding of the turbulent heat transfer in parabolic trough absorbers with pure and hybrid nano-oils, including the effect of buoyancy or gravity, is realized. This paper presents the Reynolds-averaged Navier–Stokes (RANS)-based turbulent heat transfer analyses in a 3D, long, straight for Reynolds number from 5000 to 20,000 and discrete heating conditions with different heat flux ratios such as 1, 5, 10, 20, 40, and 50 for pure oil and hybrid nano-oils having 1, 4, and 6% volume concentration of the nanoparticles. The major findings are, (a) gravity-induced anisotropy leads to high and low-speed fluid flows near the lower and upper walls, and temperature redistribution at a plane, which is beneficial, (b) the statistical axial-velocity deviates from the standard logarithmic law at a Reynolds number of 5000, and (c) the ratio of surface-area-averaged Nusselt number between the lower half and upper half of the tube is 4–12. Some important recommendations are (a) the effect of gravity must be included, (b) the local Richardson number may be used for improving the standard logarithmic law for the axial velocity, and (c) Nusselt number correlations are deduced for the upper half surface and lower half surfaces. The findings, albeit for limited parameters, will be useful for improving the heat transfer aspects in the parabolic trough absorber.