Considering numerous issues due to asphaltene precipitation during production and transportation of crude oil, this work aims to postpone the onset point of asphaltene and improve the stability of its colloids in heavy (S) and light oil (D) samples. For this purpose, water/oil interfacial tension (IFT) is measured to evaluate the effect of surfactants as asphaltene inhibitor on heavy and light oil samples from Iranian south fields. Onset of precipitation from oil samples is revealed by sudden change in IFT between deionized water and oil samples through addition of different fractions of n-heptane as precipitant. Surfactants are used to stabilize the asphaltene colloids and prevent asphaltene precipitation from crude oil. However, crude oils of different properties require investigations for appropriate surfactant at optimum concentration to delay asphaltene precipitation. In this work, three surfactants Linear dodecyl-benzene-sulfonic acid (L–DBSA), dodecyl resorcinol (DR) and nonylphenol (NP) are examined for their retarding capabilities of onset point of asphaltene precipitation in oil samples S and D. Also, the involved mechanisms for addition of surfactant to crude oil are analyzed and interpreted. Enhanced stability of crude oil and degree of postpone in onset point follows the order of L–DBSA > DR > NP for heavy oil S and NP > DR > L–DBSA for light oil D owing to nature and chemical structure of surfactants and also properties of crude oil samples. Furthermore, IFT fluctuations for light and heavy oil are similar but the faster onset point and different values of IFT for light oil arise from unstable nature and density difference.