Compressed natural gas (CNG) is a promising alternative fuel due to several main reasons, specially the strict engine emission regulations all over the world. This has made that lot's of cities have decided to use CNG as an alternative fuel in their urban transport fleets or in other urban tasks.Nevertheless, due to the recent implementation of the CNG technology in automotive sector, several problems related to lubrication have been detected, mostly affecting a reduction of the oil drain period and these problems showed no relationship with a particular fleet nor with the lubricant's brand used. These effects will have a very important weight on fleet manager's decision to select CNG as an alternative fuel, thus this reduction does not only increase the cost in engine oil, there are other maintenance actions referred to this basic period of oil drain, thus also increases other more significant costs.In this study, a representative sample of urban buses, powered with different CNG and Diesel engine technologies and working on urban duty operation, have been studied in a comparative way in order to evaluate engine oil performance differences. Main parameters evaluated have been related with oil degradation: oxidation, nitration, additives depletion, viscosity variations, and AN. Furthermore, oil refills has been monitored in order to take into account the derived effects in oil performance.After more than 190 samples of 33 vehicles (29 GNC and 4 Diesel), results obtained for selected parameters have shown higher degradation rates for oils used on CNG engines. Part of this deviation can be explained taking into account factors related with higher stress suffered by the oils used on CNG engines, mainly related with higher bmep, higher temperature and lower oil sump volume.The combination of large effects of oxidation on CNG engine oils and perhaps, low requirements demanded by engines manufacturers, make that the original oil drain period defined by those manufacturers would be impossible to reach, conducting to an oil drain reduction that will affect, in a serious manner, the costs related with maintenance actions over the engine and the vehicle and leading to a serious handicap for this engine technology compared with Diesel engines.