Abstract: Process of preparation of fluid overbased, particularly highly overbased, magnesium-containing colloidal dispersions wherein a partial mixture is formed of an oil-soluble sulfonic acid as such or admixed with an oil-soluble aliphatic hydrocarbon monocarboxylic acid, a volatile aliphatic or aromatic or chlorinated hydrocarbon solvent, a special grade or readily reactive form of magnesium oxide for neutralization and overbasing purposes, and an alcohol, carbonating the mixture with a controlled limited amount of carbon dioxide, in the presence of an added activator, adding a nonvolatile diluent oil, stabilizing the colloidal dispersion, removing undesired sediment and then heating to distill off the volatile materials.