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Atlantic Richfield Co. and Lukoil, Russia’s largest oil company, formalized their previously announced $5-billion, 18-year joint venture, called Lukarco, for oil and gas development in Russia and other countries. . . . Apple Computer Inc. was dealt another blow when Microsoft Corp. announced it would halt development on a Windows NT operating system for the PowerPC microprocessor, the engine for Apple’s Macintosh computers. . . . Informix Software Inc. has sued Oracle Corp., claiming its rival stole programming secrets by hiring key members of the Informix research team. . . . FFederal regulators gave formal approval to the $7.5- billion merger between Time Warner and Turner Broadcasting System.

Stephen W. Prough said he’s stepping down as chief executive of Downey Savings to pursue other opportunities. The thrift named veteran banker James W. Lokey to succeed Prough as president and chief executive of Downey and its parent company, Downey Financial Corp. . . .