If you use it in the oil- no problem. If you put it in fuel, it does a number of questionable things, such as put a heavier unburned HC load on the catalyst, CAUSE carbon build-up on exhaust valves, lower effective octane, etc. AFAIK, it doesn't have anything that will outright poison a catalyst (tetraethyllead, methyl manganese tricarbonyl, other metallic compounds), but the added HC load doesn't help it.