All 67 observed signals from monomethyl-substituted alkanes and mono-n-alkyl-substituted cyclopentanes and cyclohexanes were assigned in the C-13 NMR spectra of oil fractions of saturated hydrocarbons that form adducts with urea. The alpha, beta, gamma, delta, epsilon, and xi effects of a methyl substituent for chemical shifts of carbon atoms in the chain were calculated. It was shown that 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9-, and 10-monomethyl-substituted alkanes can be identified at the maximum attainable spectral resolution of these fractions, quantitative measurements being presently unrealistic only for the two last members of the series. The main distinctions in the composition of the oil fractions under study were different concentrations of n-alkylcyclopentanes.