The aim of this paper is to provide original experimental data on the viscosity of five synthetic crude oils and of three stock tank oils. This work completes a series of measurements on eight synthetic crude oils that have been published previously [A. Werner, F. Behar, J.C. de Hemptinne, E. Behar, Org. Geochem. 24 (10-11) (1996) 1079-1095]. All fluids are composed of dissolved gases (except the stock tank oils), light and intermediate hydrocarbons and a heavy oil fraction containing resins and asphaltenes. A large range of petroleum fluids, from source rock to reservoir fluids are thus modelled. The study of crude oil viscosities first requires the investigation of the phase behaviour in the same operating conditions. Seven synthetic crudes precipitated asphaltenes due to their elevated gas and low aromatics concentration. We observed at our experimental conditions of 35 MPa and 100 degrees C an increased risk of asphaltenes precipitation for petroleum fluids containing more than 28 wt.% of gas and less than 25 wt.% of aromatics. Viscosity measurements cover a range of more than two orders of magnitudes from 0.29 to 67.5 mPa s. The crude oil viscosity depends strongly on composition, especially on the gas and asphaltene concentrations.