To obtain an inexpensive additive capable of improving the solubility to oil and preservability of aromatic hydroxycarboxylic acid alkaline earth metal salts, easy to recover in distillation process, and useful for e.g. lubricating oil cleaning agents, containing a higher aliphatic carboxylic acid, an amine or amide.

This additive contains (A) a higher aliphatic carboxylic acid (e.g. 16-36C aliphatic carboxylic acid) and (B) a higher aliphatic amine (e.g. 16-36C aliphatic primary amine) or (C) a higher aliphatic amide (e.g. 16-36C aliphatic amide. This additive is added at 0.05-1.0mol, per mol of the alkaline earth metal in an aromatic hydroxycarboxylic acid alkaline earth metal salt (e.g. bearing 8-36C hydrocarbon side chain and free from any alkali metal). It is preferable that this additive is added at 0.01-0.5wt.% and 0.01-1.0wt.% to a diesel fuel coil and a gasoline fuel for two-cycle engines, respectively, and at such an amount as to give a base number of 5-80mgKOH/g to a lubrictating oil.