Historically, the anti wear additive used in the majority of oils was a compound known as ZDD (Zinc Dialkyl Dithio Phosphate). The phosphorous content of ZDDP and its ash, has a detrimental effect on catalytic emission systems. This has led to new oils being developed to meet modern regulations, which have reduced levels of ZDDP. These new oils may cause serious problems in a classic engine. Add one bottle of ZDDP at each oil change to protect the gears, cam and followers and add life to your engine. Note: Millers Classic Oils still contain sufficient ZDDP.