An internal combustion engine operating on compressed natural gas (CNG) as fuel is usually under higher thermal stress compared to the same engine using gasoline fuel. This leads to various concerns on the operation of CNG engine and the performance of the lubricant, such increased wear, accelerated total base number depletion, and faster deterioration of the lubricant. Engine oil intended for compressed natural gas (CNG)-gasoline bi-fuel passenger car application must therefore be formulated to withstand the varied and often severe operating conditions, as well as maintain superior lubrication control and prolong engine life.PTT Public Company Limited (PTT) has developed a new CNG-gasoline lubricant meeting API SN/GF-5 performance category that is able to address the various operating concerns of bi-fuel passenger car engines, and at the same time provides extended oil drain interval (ODI) capability.This formulation development comprised of two rigorous field trial phases conducted in Bangkok, Thailand. The first phase involved the screening of various candidate formulations developed by PTT in a fleet of high mileage bi-fuel taxi vehicles. A final candidate formula was selected for further proof of performance in the second phase controlled field trial. This second phase controlled field trial was carried out on five new bi-fuel vehicles and two new conventional gasoline vehicles, and at an extended ODI of up to 25,000 km as compared to original recommended 10,000 km ODI. The engines of 3 field trial vehicles were inspected at the end of trial. The CNG-gasoline bi-fuel engines were observed to in excellent condition with minimum sludge & deposit issue, as well as an excellent used oil analysis (UOA) result throughout the trial. Despite the trial was run at extended ODI, the nitration/oxidation number and total acid number (TAN) growth/total base number (TBN) depletion were well-controlled. In addition, the candidate formula exhibited minimal engine wear, superior piston deposit ratings, as well as rendering potential fuel economy benefit. The candidate formula has shown to exceed the performance requirement for the latest CNG-gasoline (bi-fuel) passenger cars, and even for high mileage bi-fuel taxi vehicles under severe driving conditions in Bangkok, Thailand.