Power transmitting fluids containing esters of hydrocarbyl succinic acid or anhydride with thio-bis-alkanols

A process for improving the friction modification properties of a power transmission fluid containing at least one friction modification agent and a mineral oil base, which comprises employing as at least one of said friction modifying agents, at least one ester additive formed by the reaction of:

• (a) a thio bis-alkanol of the formula:
• ##STR4## wherein R and R' each independently may be hydrogen, methyl or ethyl, x may be 1-4, and a, b, c, d, each may be independently 1-3, with (b) 1 to 2 moles, per mole of the thio-bis-alkanol, of an aliphatic hydrocarbon-substituted succinic acid or anhydride wherein the aliphatic hydrocarbon group contains from 12 to 50 carbon atoms, the additive being present in an amount effective to provide friction modification, corrosion inhibition, anti-wear, oxidation inhibition or extreme pressure properties to said fluid.