In Thailand, most heavy-duty trucks were equipped with diesel engine, while a small portion was equipped with compressed natural gas (CNG) engine. However, in the past few years the number of CNG fuel trucks in Thailand has increased significantly due to the cheaper cost of CNG. In general, the emphasis of heavy-duty diesel engine oil performance is on piston cleanliness and soot handling properties, while thermal and anti-oxidation properties are most critical for CNG engine oil performance. For truck fleet owners who operate both types of trucks, using the inappropriate oil that is not fit-for-purpose can adversely affect engine performance and reduce engine service lifespan under prolonged usage. A novel CNG/diesel engine oil has developed by PTT to meet both JASO DH-2 performance for heavy-duty diesel engine oils and OEM requirements for CNG engine oils. The candidate formulation was proved adequately fit for practical use regarding to thermal and anti-oxidation properties, as the result of Komatsu hot tube deposit bench tests, ISOT and NOx bubbling test. Furthermore, engine durability tests were conducted for 4 litters and 8 litters of diesel engines with 8 litters of CNG engine. Top Groove Fill (TGF), Weighted Total Demerit (WTD) as piston cleanliness and used-oil properties were measured. A field-trial was conducted on commercially operated in-service trucks consisting of 5 diesel and 3 CNG engines for a total mileage of 30,000 km in a tropical climate under real world driving conditions in Thailand. The developed SAE 15W-40 viscosity grade engine oil exhibited no harm in both diesel and CNG trucks throughout the entire field-trial.