Oil compositions comprise a molybdenum salt of a di-esterified phosphoric or thiophosphoric acid, of general formula: <FORM:1016832/C4-C5/1> wherein the R groups are hydrocarbon or substituted hydrocarbon which may be the same or different, X is sulphur or oxygen, a and b are each a positive integer, and (a+2b) is the valence of molybdenum and is from 6 to 8 (see Division C2), together with gasoline, fuel oil, lubricating oil, grease or power transmitting fluid. R is preferably C3 to C30 alkyl, such that the average chain length of the hydrocarbon groups exceeds 7.6, rendering the compound soluble in hydrocarbon oil compositions. Lubricating oil composition may contain animal, vegetable, mineral or synthetic oils. Examples of the latter specified are dodecyl adipate, and di-2-ethylhexyl sebacate. The concentration of molybdenum salt may be from 0.001% to 10% by weight. A composition of S.A.E. 30 mineral oil with 0.89% by weight of O,O1-dinonyl-phosphorodithioic molybdenum oxide is mentioned. Other additives may also be present, e.g. detergents, both ashless and metal containing, oxidation inhibitors, film strength agents, oiliness agents, pour point depressants, foam inhibitors, and viscosity index improvers; named examples of all these additives are given.ALSO:The invention comprises molybdenum salts of organic phosphorus acids, of general formula <FORM:1016832/C2/1> wherein the R groups are hydrocarbon or substituted hydrocarbon, which may be the same or different, X is sulphur or oxygen, a and b are both positive integers, and (a+2b) is the valence of molybdenum, and is from 6 to 8. R may be alkyl, aryl, alkaryl, cycloalkyl, or aralkyl and may be substituted (e.g. by -Cl, -Br, -I, -NO2, -OR, -SH, NH2, or C=O); R as alkyl with 3 to 30 carbon atoms is preferred. The diesterified phosphorothioic acids may have up to four sulphur atoms, and may be prepared by conventional methods (e.g. reaction of P2S5 with a suitable alcohol at 50-200 DEG C., reaction of mercaptans or thiophenols with PSCL3 or PSBr3 to give an intermediate which is reacted with H2S, and replacement of sulphur by oxygen by reaction with steam or water). The molybdenum salts may be prepared by the reaction of the organic phosphorus acid with an alkali metal molybdate or thiomolybdate, or with a suitable molybdenum sulphide or oxide such as the trisulphide, tetrasulphide or trioxide; the latter reaction is promoted by water or a lower alkanol. Alternatively, the alkali metal salt of the phosphorus acid diester may be reacted with alkali metal or ammonium molybdate, suitably in an organic solvent. Examples describe the preparation of: tetra-[O,O-di-(4-methyl - 2 - pentyl phosphorodithioic]molybdenum oxides of varying Mo content; O,O-dinonyl phosphorodithioic substituted molybdenum oxide; tetra-[O,O-di-(2-ethyl-hexyl)-phosphorodithioic]molybdenum oxide; and O,O - di - (p - tertiary - pentyl phenyl) phosphorodithioic molybdenum oxide. The compounds are useful as emulsifying agents, detergents, plasticizers, paint driers, corrosion inhibitors, anti-foam agents, oxidation inhibitors, and, when soluble in hydrocarbon compositions, are useful as lubricating additives in lubricants and fuels (see Divisions C4 to C5).