Synthetic, semi-synthetic and mineral vehicular oils used in gasoline/alcohol/VNG engine were evaluated by Rancimat® method. The experiment consisted in submitting 3 g of different virgin (off the self) motor oils samples to a temperature of 110°C and 10 L/h air flow. The oils are chosen and classified as multiviscous brands and recognized by their trademarks: Lubrax (Petrobras) and Mobil (Esso). The conductivity curves show the oxidative evolution as a function of time for the different oils. The synthetic, as well as, the mineral motor oil shows almost the same profile, where the conductivity curve differential is about 0.55 μS/cm₂h. However, once one start measuring up to 10.8 days, the mineral oil presents an oxidation meaning values 1.4 μS/cm₂ higher than any synthetic oil.