The rheological behaviour of the fluid which transmits the pressure is one of the most critical points in the design of high-pressure standards. A compromise must be found between its properties under low and high pressures. Unfortunately, the influence of pressure on the viscosity of liquids is very strong, roughly exponential, and this reduces the effective pressure domain covered by each liquid. This paper first presents results and comparisons with previous work. The second part deals with methods, standardized tests and the equipment required to select and to test a lubricant. The third section introduces new potential fluids.