A. If the natural gas engine is spark ignition and operates totally on compressed or liquefied natural gas, major engine manufacturers recommend a natural gas engine oil as opposed to an oil designed to operate on diesel fuel engine and compression ignition. There are engines that can be set up to operate on dual fuel which allows the engine to run on natural gas or diesel fuel. These engines are compression ignition and utilize conventional diesel engine oils such as API CJ-4 engine oils. Cummins Westport, for example, does require that a natural gas engine oil must be used in their spark ignition engines. Cummins Westport lists these engines oils as complying with the Cummins--CES 20074 specification. It is equally important as in all cases to know the specific manufacturers recommendations.