The smoke at the start up of a car is due to some oil in the combustion
chamber which seeps from the valve guides. As you park your car for some
hours, the engine oil which collect in the tappet area get slowly seeped into
combustion chamber through damaged valves guide and your car will smoke
only for few minutes untill the oil in the combustion chamber will burnt.
please replace valve guides and the problem will be solved. Your mechanic is
not an expert. It is not due to mixing or oil in CNG or poor piston rings.