A properly understanding of how oil mixtures behaves as they flows in a pipeline is a key factor for piping design. Facilities engineers from oil companies currently use commercial pipeline simulators in order to perform pressure and temperature gradients predictions. Nearly all the simulators apply the general mixing rule when they need to calculate the viscosity of a certain oil mixture. As viscosity depends both on molecular characteristics and molecular interaction, and oil is a complex mixture of components, it is not surprising that this rule gives poor results. Besides, non-Newtonian behavior makes things more complex. This paper discusses experimental results of mixing at lab several different oils and measuring their viscosity. Each oil had its own composition and rheological behavior. Some mixtures were prepared in order to reproduce real situations. Once the mixtures viscosities were obtained they were introduced as input data in a commercial pipeline network and nodal analysis software. Then, pressure and temperature gradients were calculated a) Using the experimental mixture viscosity b) Using the general mixing rule viscosity estimated by the software Results showed, in some cases, dramatic differences between the two alternatives. Higher deviations were obtained where the mixtures were composed of crudes of different properties. In order to improve the accuracy other mixing rules were tried and some of them fit better. A proper understanding of crude oil properties seems to be essential for successfully applying complex simulators in pipeline design