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Catalyst poisoning from engine oil additives is a complicated process that depends in part on the pathway by which the oil is consumed in the engine. Engine studies were conducted to assess the relative impact of three major modes of oil consumption - through the PCV system, past the piston rings, and through the valve guides. Minimal phosphorus poisoning was observed with oil consumed through the PCV system and piston rings, whereas oil consumed through the intake valve guides demonstrated severe catalyst poisoning. The former produces effects characteristic of complete combustion of the ZDDP additive previously shown to produce relatively innocuous washcoat overlayers of porous zinc phosphate. In contrast, the latter produces effects characteristic of incomplete combustion (i.e., spray of oil additive into the exhaust and, most notably a washcoat pore-plugging effect accompanied by a marked decrease in washcoat surface area.