A rust and corrosion inhibiting marine diesel engine lubricating oil additive is provided which comprises a mixture of:(a) an oligomeric polythiocyclic imide represented by the formula ##STR1## wherein R and R.sup.1 each are a (C.sub.1 -C.sub.50) saturated or unsaturated aliphatic group and n is an integer of 0 to 5; an oligomeric amine represented byR"--NH.sub.2where R" is dodecyl or a (C.sub.12 -C.sub.25) alkyl group; and either(b) a polymeric additive of a poly [ethylene-co-propylene)-g-succinic anhydride] represented by the formula ##STR2## wherein ethylene (E) and propylene (P) are randomly incorporated; a and b correspond to 60 mole % and 40 mole %, respectively; and the repeat unit, c, is an integer of 40 to 40,000 so that the overall molecular weight of said succinic anhydride is 50,000 to 70,000 AMU's and an(c) amino thiocyclic represented by the formula ##STR3##