The aim of this work is preparation and characterization of nanofluids with two features most desired for heat transfer systems: long term stability and high thermal conductivity. For this purpose, suspensions of multi-wall carbon nanotubes (MWCNTs) / graphite were prepared in a base oil fluid PAO 8 with the addition of surface-active substances (surfactants). In the first phase of the experimental work, it was determined which of surfactants, for PAO 8 oil, provides the greatest stability of nanofluids. In the second phase, the effect of different concentrations of filler and selected surfactant (DPS) on system stability was studied. Stability of prepared nanofluids was determined by sedimentation test (visually) and by UV-Vis spectrophotometry. Characterization of nanofluids is accomplished by determining the viscosity, thermal conductivity, wear resistance and the contact angle at the solid-liquid-gas interface. The surfactant, that had enabled the greatest stability of nanofluids, was dispersant polymer additive based on styrene and methacrylate (DPS). By increasing its concentration there was an increase in viscosity of nanofluids, and thus greater system stability (Stokes' law) was accomplished. The only increase in thermal conductivity was noted in nanofluid containing 2600 mg of MWCNTs, because the heat transfer takes place only via nanoparticles in touch. The wearing test results indicate a reduction of mean wear diameter of all nanofluids, which indicates better lubricating properties. The results of the contact angle measurement indicate that the interaction between the sample and teflon surfaces are not favorable, while glass and metal surfaces had lower contact angle with samples. Results for metal surface indicate decreasing trend of the contact angle with the increase of carbon nanotubes. From the standard deviations for all three cases, it can be seen that the method has a big deviation and it is not possible to draw valid conclusions.