A few years back, apparently, the standard oil mixtures were reformulated to remove some components vital to the proper operation of older automotive engines. A ZDDP Additive can be found at Eastwood under the lable ZDDPlus.
ZDDP Oil Additive
The change of formula is represented in the API. API SL is good for older cars but API SM is not as it has been reformulated to lower the ZDDP/zinc levels.