An adapter apparatus is provided for use in mounting two oil filter cartridges on an engine. The adapter apparatus includes a manifold for interconnecting the oil filters, and for allowing oil to flow in series through the oil filters and an engine. The manifold includes a metal manifold block having an outer face, an inner face, and at least one lateral edge with a plurality of cooling fins thereon. The manifold block has a plurality of hollow flow passages formed therein to route oil therethrough. Two sleeve fittings are attached to the manifold block, and each of the sleeve fittings has male threads formed on an outer surface thereof. Each of the sleeve fittings threadably receives one of the oil filters thereon. The apparatus also includes inlet and outlet tubes attached to the block body, for respectively routing oil to and away from the adapter apparatus.