Abstract: A railway diesel crankcase lubricant composition comprising a major portion of a diesel lubricating oil and a minor amount of, as an oxidation and corrosion inhibiting additive, a condensate product prepared by the process comprising:(a) reacting a dibasic acid anhydride with an oligomeric isobutylene to form a succinic anhydride;(b) reacting the succinic anhydride with an N-alkyl alkylene diamine to produce a succinimide-amine;(c) reacting the succinimide-amine with a polyaromatic diisocyanate to produce a phenylurea isocyanophenylmethane;(d) reacting the isocyanophenylmethane with a heterocyclic compound containing a thiadiazole nucleus to a condensation product of a mercaptothiadiazole-phenylmethane; and(e) recovering the condensation product.