Unocal Corp., joining a growing contingent of oil companies with clean air ideas, announced Thursday it would offer to buy 7,000 older gas-eating cars in an effort to reduce smog in the Los Angeles basin. Unocal has allocated $5 million to purchase the cars, which must be at least 20 years old, and scrap them, said Richard J. Stegemeier, Unocal's chairman and chief executive. The plan would eliminate an estimated 6 million pounds of pollutant gases, he said. Owners would be offered $700 apiece for the locally registered junkers, Stegemeier said. ``We want to take the heaviest polluters off the road and hand them over to a scrap yard, which will shred and recycle the metal,'' Stegemeier said. Unocal executives noted that pre-1971 vehicles pollute from 15 to 30 times more than 1990 models, with about 410,000 such jalopies and bombs registered in the region. The program is scheduled to begin June 1. Another voluntary program, to take effect in July, offers free smog checks and emission control tune-ups for pre-1975 cars at participating Unocal 76 service stations. Such tune-ups could remove 17 million pounds of carbon monoxide, nitrogen oxides and reactive organic gases from the Los Angeles basin, which has the worst air quality problem in the nation, the Los Angeles-based oil company said. A spokesman for Southern California's powerful air quality agency voiced support for Unocal's program, which was seen by one industry analyst as a growing trend of ``greening'' among oil companies. Unocal's approach also reflects a different philosophical approach to eliminating air pollution in the region. ``We want to demonstrate ways of reducing smog that are efficient and cost-effective,'' Stegemeier said. A similar proposal was suggested last year to regional smog control officials by the Claremont Institute, a conservative non-profit think tank. The institute had suggested paying about $1,500 apiece to remove 850,000 pre-1980 vehicles from the region, at a cost of $1.3 billion, said Lucas Morel, a research associate at the institute. It moves away from the command and control of a centralized bureaucracy by allowing industries in the marketplace to choose their own methods to achieve certain reductions of emissions, Morel said. While unusual in its approach, however, Unocal's announcement was hardly unique. ``They're all becoming environmentally pro-active,'' said Kenneth B. Funsten, an oil industry analyst with Wedbush Morgan Securities in Los Angeles. Funsten attributed the trend largely to Exxon's handling of the Alaskan oil spill, which has hardened political opposition to oil industry operations. Atlantic Richfield Co. last September introduced a reformulated low-emission gasoline to replace leaded regular in its pumps, and the Shell Oil Co. followed suit with a similar reformulation for premium gas.