Overview: The phosphorus content of new oil is typically associated with zinc dialkyldithiophosphate antiwear  additives (ZDDP).  Antiwear additives are used to reduce wear by limiting the friction of moving parts. They do so by forming a thin protective film on metal surfaces.  Low phosphorus could indicates that there may not be enough anti-wear protection of the oil, however this could be compensated by adding a non-phosphorus containing aniwear additive  High Phosphorus levels can lead to emission system failures due to catalytic converter  poisoning The current API standards for  passenger car engine oils (API SM, SN, and ILSAC GF-5) require phosphorus to be between 600 to 800 ppm to provide sufficient wear protection while guarding against catalytic converter poisoning (shown by the area in green).  The amber area takes into account the precision of the test method.  Results in the red area do not meet current API passenger car motor oil standards.