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 was developed to meet both JASO DH-2 heavy-duty diesel engine oil performance and CNG engine oil performance. The candidate formulation was proved adequately fit for practical use regarding to thermal and anti-oxidation properties. Engine durability tests were conducted for 4-liter and 8-liter of diesel engines with 8-liter of CNG engine. Top Groove Fill (TGF), Weighted Total Demerit (WTD) as piston cleanliness and used-oil properties were measured. It was further demonstrated that the Komatsu hot tube deposit bench test correlated well with 8-liter diesel engine durability test. A field-trial was conducted on commercially operated diesel and CNG trucks 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.