A natural gas engine oil must be designed and balanced in a proper way in order to be able to offer: excellent deposit control, superior oxidation and nitration control, extenden lubricant filter and engine life and reduced frecuency of shutdowns for main tenance.In order to choose a correct formulation, it was decidet to run a field tests where five differents low ash additive packages were tested. The selected engines are five WAUKESHA, model H2475, they are rated at 300 HP and are used for water pumping. The five engines were rebuilt before the beguining of the test. The liner diameters were measured, the same as the ring gaps. At the end of the test, that lasted 10.500 hs., the measurements were repeated and pistons and cranckases were inspected and calificated for deposit formation. Each 700 hours, a lubricant sample was analyzed for its main characteristics in order to evaluate its performance.