DUMPING used motor oil down the nearest storm drain or putting it out with the trash have long been practices of many do-it-yourself oil changers. The Environmental Protection Agency (EPA) estimates that Americans improperly dispose of 90 percent of the 200 million gallons of oil they change each year.Growing sensitivity to the need to protect ground-water quality is prompting increased collection of used oil. And that will help companies that profit from hauling it away and recycling it. Various localities already are making the effort. Florida launched a state-wide oil recycling program in 1988 by giving grants to communities to establish collection centers. Now the effort is going national. Both Mobil Corporation and Exxon Company USA have recently announced that 3,000 or more service stations will act as drop-off points for used oil. "We're still out signing them up right now," Exxon spokesman Henry Beathard says. Mobil says its program will be fully in place by the end of this month. It expects most of its 3,300 dealers as well as some distributors to participate. The two oil giants had already been operating such programs successfully in selected markets. Exxon began with test programs in Baton Rouge, La., and Richmond, Va. It found that motorists delivered an average of 1,000 gallons of oil a year to each station. If that holds true in the expanded program, Exxon will collect 3 million gallons, or 1.5 percent of the oil motorists now throw away. But the company's projection for the program's first year is only 2 million gallons. Although in some locations the oil can be sold as boiler fuel, Exxon has no illusions of making money from the program. "It's a cost item, not an income item," Mr. Beathard says. Participating service stations will spend an average of $100 to have the collected oil hauled away. Exxon intends to pay that amount as a flat fee to the stations, regardless of volume, to eliminate paperwork. But that $300,000 annual cost won't be the only expense. Exxon will promote the program through advertising and signs at its stations. And consumers will be able to call an 800 number to find the nearest Exxon drop-off point. Stations must also be equipped with a contaminant sniffer to test oil before accepting it from motorists. Beathard says oil can be contaminated by chlorine-based solvents such as are found in carburetor cleaning sprays. In its test markets, Exxon had to reject 3 to 5 percent of the oil that was brought in, says David Allan, the project's coordinator. That will probably drop to 2 to 3 percent later, since in the early stages of the program people bring in oil that has been sitting in their garages and has been exposed more to contaminants. The issue greatly concerns service station operators, who under the federal Superfund law could be held responsible for cleanup costs if they passed along contaminated oil to a recycler who later went bankrupt and left behind a toxic mess. That has happened, says Mike Carney, senior vice president of marketing at Safety-Kleen Corporation. The Elgin, Ill., company recovers and recycles a variety of fluids from industry. Motor oil has not been designated a hazardous waste by the EPA, although the agency is thinking about doing so. But Safety-Kleen considers motor oil a hazardous waste, and has built its facilities accordingly. Thus the company can assure those whose oil it takes that there's no need to worry about Superfund liability. The company has a used-oil re-refinery at Breslau, Ontario, and just finished building another plant at East Chicago, Ind. The new plant will eventually be expanded to a capacity of 75 million gallons a year. Safety-Kleen collects 100 million gallons of used oil in the eastern US, making it the nation's largest recycler of oil. Some 60 percent of the oil is turned back into lubricant blending stocks, while the rest becomes fuel or road-building material. The East Chicago plant will allow more oil to be turned into lubricants. Service stations and other businesses that change motor oil pay Safety-Kleen $50 to haul away 200 gallons of oil. Nationwide, such businesses generate 500 million gallons of used oil per year.