Abstract: This invention is a process for improving the rheological properties of oil well drilling fluids particularly useful for oil-based invert emulsion types of drilling fluids. The process is particularly useful when drilling in deep water from an off-shore drilling rig. The new process uses as a rheological viscosifier, for such fluids, a composition which is a mixture of organoclays and castor wax and/or castor was type rheological additives, which when added to a drilling fluid at from about 0.5 and 5% by weight, creates an inventive drilling fluid compositions less sensitive to the variety of different and varying, very hot and very cold temperatures found in the drilling hole, and in the long stem of drilling pipe. The present invention also relates to the discovery of oil based invert emulsion drilling fluids that exhibit greater efficiency and provides more stable drilling fluid viscosity and anti-settling performance over varying temperatures when compared to conventional fluids containing organoclays.