A diesel lubricant exhibiting improved ability to minimize undesirable viscosity increases when used in diesel engines. More particularly, in accordance with the present invention, a diesel lubricant is described which comprises a major amount of an oil of lubricating viscosity and a minor amount, sufficient to minimize undesirable viscosity increases of the lubricant when used in diesel engines, of a composition comprising (A) at least one carboxylic derivative composition produced by reacting at least one substituted succinic acylating agent with at least one amino compound containing at least one -NH- group wherein said acylating agent consists of substituent groups and succinic groups wherein the substituent groups are derived from polyalkene characterized by an Mn value of at least about 1200 and an Mw/Mn ratio of at least about 1.5, and wherein said acylating agents are characterized by the presence within their structure of an average of at least about 1.3 succinic groups for each equivalent weight of substituent groups, and (B) at least one basic alkali metal salt of at least one acidic organic compound having a metal ratio of at least about 2. The diesel lubricant also may contain (C) at least one oil-soluble neutral or basic alkaline earth metal salt of at least one acidic compound. Preferably, the basic alkali metal salt (B) contained in the diesel lubricants of the invention is at least one sodium or potassium salt and more preferably, a sodium salt of a sulfonic acid. Optionally, the diesel lubricants of the invention can contain (C) at least one neutral or basic alkaline earth metal salt of an acidic organic material. The invention also includes methods for preparing the alkali metal salts, particularly the potassium salts, and methods for operating diesel engines which comprises lubricating said engines during operation with the diesel lubricants of the invention.