The following are engine oil suppliers that offer oils formulated for CNG, LNG, and propane powered engines.
Valvoline Premium Blueョ GEO Engine Oil, Mobil GEO 15W-40, AMSOIL Synthetic Vehicular Mobile Natural Gas Engine Oil SAE 15w40, Shell Rotella T3 NG 15W-40, etc.
While these oils are for heavy duty applications they are required because CNG, LNG, and propane create more ash during the combustion process. There is a long list of oil manufactures and explanations available on the internet as to why CNG, LNG, and propane engines require a low ash oil. Westport Engineering and other engine manufacturers offering CNG, LNG, and propane powered engines may offer more technical information about this issue.
Regards,
HotRod1