In the present work, the influence of temperature and concentration on the viscosity index of oil-multiwalled carbon nanotube (MWCNT) nanofluids has been investigated theoretically and experimentally. Data were collected for temperatures ranging from ambient to 100C and for concentrations ranging from 0.01 to 0.2 wt.% of MWCNT. The results show that viscosity is enhanced with increasing the MWCNT concentration and decreasing temperature. Experimental results emphasize that the maximum enhancement of the viscosity index is 14.11% for MWCNT-oil nanofluid. Stability examinations of the nanofluids have been performed by a UV-vis spectrophotometer. It has been found that the Einstein formula and those derived from the linear fluid theory are valid for relatively low particle volume fractions of MWCNT, and for higher MWCNT concentrations the discrepancy between these formulas and experimental data is significant, indicating that the linear fluid theory is no longer appropriate to represent the real behavior of these nanofluids.