Method of preparing overbased lubricating oil additives

A method for preparing an overbased lubricating oil additive, which comprises the steps of:

• I. forming a mixture consisting essentially of (1) an oil-soluble organic acid, or a metal salt thereof, wherein said organic acid is selected from the group consisting of hydrocarbon carboxylic acids and hydrocarbon sulfonic acids wherein the hydrocarbon group has from 8 to 150 carbon atoms, and (2) a compound selected from the group consisting of alkaline earth metal oxides and alkaline earth metal hydroxides, in a lubricating oil as a reaction medium, the metal ratio being in the range of from 2 to 15,
• Ii. adding to said mixture an accelerator composition consisting essentially of
• (a) from 0.5 to 1.2 moles of a dihydric alcohol having from 2 to 6 carbon atoms, per one mole of component (2) in said mixture,
• (b) from 0.01 to 0.2 moles of an alkyl phenol having from about 4 to 100 carbon atoms in the alkyl groups, per one mole of component (2) in said mixture, and
• (c) from 0.1 to 0.6 gram atoms of elemental sulfur, per one mole of component (2) in said mixture,
• heating the mixture to a temperature above 120.degree. C and effective to cause the reaction in the liquid phase, until the water formed by the reaction is distilled off;
• and then
• Iii. blowing carbon dioxide gas into the reaction mixture obtained in step II, at a temperature above 100.degree. C, to transform at least part of the excess of alkaline earth metal into the corresponding carbonate, and distilling off the volatile substances from the reaction product to obtain the overbased lubricating oil additive.