Here is a response to my question regading a Zinc additive.
Hi John;
Thank you for considering Lucas Oil Products. As for your question YES you should most defiantly be using a Zink additive. Zink is a high pressure additive that was recently removed from both synthetic and conventional oils. This was done because over prolonged usage it could harm your catalytic converter. Your 67 273 dodge dart and 68 390 ford F350 were not designed to run without Zink and you will risk cam failure or push rod failure. A full bottle of our TB-Zink Break-in Oil Additive should be added to every oil change. In a five quart application it will bring the Zink content to aprox 5200 PPM which is more than enough to protect your engine. Since Zink is a additional Lubricant in your oil it does not make sense that it would increase engine ware. As for synthetics Lucas TB-Zink is designed to be used with both synthetic and conventional oil. I have attached our Technical Data Sheet. Please disregard the part about not designed for passenger car use as this is referring to cars with Catalytic Converters and also the EPA does not want you putting Zink back into the oil so we have to say that.
http://www.lucasoil.ca/products/product.asp?id=67&cat=Automotive
