The invention provides a method employing a fuel additive to improve the operation of a pass-through oxidation catalyst over long periods of time with continued catalytic activity and without the undesirable conversion of SO2 to SO3. A pass-through catalyst support is continuously, selectively catalyzed by the combustion of a fuel containing a platinum group metal composition which burns to release the catalyst metal in active form. The effectiveness of the process is attributed to improved combustion in the engine by the catalyst and the treatment of the catalytic oxidizer with active catalyst metal released during combustion such that, in the exhaust system, the soluble organic fraction of the particulates and the levels of gaseous unburned hydrocarbons and carbon monoxide are significantly reduced. The catalysts conditioned by the present invention cause minimal conversion of SO2 to SO3. The platinum group metal compositions soluble or dispersible in the diesel fuel and are added in amounts effective to provide concentrations of the metal in the fuel of less than 1 part per million (ppm).