The major phosphorus-containing compounds in engine oil are zinc dithiophosphates (ZDP's), which act both as antioxidants and antiwear agents. To reduce engine oil phosphorus concentrations without compromising engine durability, and thereby reduce phosphorus poisoning of emission control devices, an optimum ZDP or ZDP mixture should be used. A 160 000-km taxi test was conducted to determine the relative camshaft and lifter wear protection provided by several ZDP's and ZDP mixtures. Wear protection was poorest with aryl ZDP's (which are thermally stable, as determined by differential scanning calorimetry (DSC)), intermediate with long-chain primary alkyl ZDP's (which are thermally unstable), and best with short-to-medium chain secondary or short-chain primary alkyl ZDP's (which are of intermediate thermal stability). Sequence IIID test results on analogs of the field test oils correlated fairly well with taxi test results; Sequence V-D test results did not correlate as well.