PURPOSE: To obtain a partially sulfurized liquid hindered phenol product low in copper corrosiveness and suitable as an antioxidant for lubricating oils by reacting a liquid mixture of phenols mainly made up of a hindered phenol(s) with a specified amount of a sulfur chloride.

CONSTITUTION: A liquid phenol mixture made up of one or more reactive hindered phenols (e.g. 2,6-di-t-butylphenol and 2,4-di-tert-butylphenol) in an amount of at least about 50 wt.% is prepared. Then the mixture is reacted with sulfur chloride in a ratio of 0.3 to 0.7 gram atom per mol of the reactive phenols to recover a liquid product thereby obtaining a partially sulfurized liquid hindered phenol product. The obtained product is allowed to be contained as an antioxidant in an amount of 0.05 to 5 wt.% in an oil having a lubricating oil viscosity thereby enabling a lubricating oil for decreasing the formation of sludge for crank chambers to be produced.