Don't do it! First it'll poison your Cat. Here is a quote from Mobile Tuning:"With the addition of the catalytic converter, it is undesirable to burn leaded fuel, because leaded fuel will clog the converter and increase the back-pressure of the exhaust. "
Also may damage O2 sensors because they are sensitive to contaminants. Here is a quote from a Toyota fuel injection site: "The O2 sensor is easily contaminated, and as such leaded gasoline should never be used, as well as silicone sealants/gaskets that are not O2 safe. "