CNG burns higher temperature than Petrol. for that u need extra protection, Use any oil but no shell oils, they r the worst and oil reduces from day to day running, i would prefer Caltex CXT, which i think is Havoline now, and One tin Pack of US Oil , Oil Treatment, it thickens the oil to increase the viscosity of the oil and hence gives more reliable protection to your car. But dotn use this Oil Treatment in winters or else u would have big issues in starting up your car in the morning