A phosphorous- and sulfur-containing additive and its use to impart anti-wear and/or antioxidant properties to oleaginous compositions such as fuels and lubricating oils, particularly power transmission compositions, such as automatic transmission fluids, is disclosed. The additive comprises a mixture of products formed by simultaneously reacting (1) a beta-hydroxy thioether, such as thiobisethanol, and (2) a phosphorous-containing reactant, such as tributyl phosphite. The reaction is carried out under reduced pressure conditions (e.g., -40 KPa to -100 KPa) to limit the amount of thioether species in said additive to less than about 45 mole %.