An ester-containing reaction product of a carbonyl compound, preferably an acyl halide and a thiodiphenol has high temperature antioxidant properties. The reaction product is useful as a synthetic lubricant base fluid or as an antioxidant additive when used in minor amounts of 0.01 to 10 wt. % in a mineral oil or hydrocracked oil lubricant base fluid. The reaction product can also be used in a fuel.