(57) Copper corrosiveness of compositions comprising a major amount of a base oil of which at least 50% by volume is mineral oil that has an inherent sulfur content of at least 0.2 percent by weight is reduced by adding to the oil a small amount of at least one oil-soluble neutral hydrocarbyl phosphite ester, most preferably triphenyl phosphite. The addition of phosphite can yield composition exhibiting a copper rod rating of 5 or below in the Cincinnati Milacron Thermal Stability test procedure "A", even though in the absence of the addition of phosphite, the composition fails the test. The unpredictability of the invention is shown by the fact that not all high sulfur oils respond to the treatment, and no explanation for the divergence of results is presently known.