After 70+ years, the EPA mandated that all domestic oil producers remove ZDDP (Zinc Dialkyl Dithio Phosphate) from all conventional and synthetic motor oils. If your engine was designed before 1989 the non-roller lifters require ZDDP to avoid premature deterioration. ZDDP also alters bearing and journal surface characteristics to prevent metal to metal contact. Under extereme conditions like a high perfromance motor, ZDDP reduces the tendancy of parts to scuff or gall under heavy loads. Simply add Eastwood ZDDP Oil Additive to every 4-6 quart oil change and your motor will be protected.