Additive composition

An oil additive composition, comprising:

• (a) a compound having, in the molecule, a group of a general formula (I):
• ##STR7## wherein R.sup.1, R.sup.2 and R.sup.3 each represent a hydrogen atom or a methyl group, but at least one of R.sup.2 and R.sup.3 is a hydrogen atom,
• and (b) at least one extreme-pressure improver containing sulfur, selected from the group consisting of sulfurized olefins of formula (II),
• R.sup.4 -S.sub.a -R.sup.5 (II)
• wherein R.sup.4 is an alkenyl group having from 2 to 15 carbon atoms, R.sup.5 is an alkyl or alkenyl group having from 2 to 15 carbon atoms, and "a" represents an integer from 1 to 8;
• dihydrocarbyl polysulfides of formula (III):
• R.sup.6 -S.sub.b -R.sup.7 (III)
• wherein R.sup.6 and R.sup.7 each independently represents an alkyl or cyclic alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an alkylaryl group having 7 to 20 carbon atoms, or an arylalkyl group having 7 to 20 carbon atoms, and "b" is an integer from 2 to 8;
• thiadiazole compounds of the following formulae (IV):
• ##STR8## wherein R.sup.8 and R.sup.9 each represent a hydrogen atom, or a hydrocarbon group having 1 to 20 carbon atoms;
• and "c" and "d" each represent an integer from 0 to 8, and alkylthiocarbamoyl compounds of formula (V):
• ##STR9## wherein R.sup.10 and R.sup.13 each represent an alkyl group having 1 to 20 carbon atoms;
• and "e" represents an integer from 1 to 8, and an extreme-pressure improver containing phosphorus selected from the group consisting of formulae (VI) to (X):
• ##STR10## wherein R.sup.14 to R.sup.16 each represent an alkyl, alkenyl, alkylaryl or arylalkyl group having 4 to 30 carbon atoms, and R.sup.14 to R.sup.16 may be the same or different;
• and component (a) is present in an amount of 1 to 99% by weight and component (b) is present in an amount of 1 to 99% by weight, based on the total amount of components (a) and (b).