LAS VEGAS (AP) _ The nation's auto dealers are approaching the Big Three's quick oil-change operation offers cautiously, wondering if the investment is worth the return.

General Motors Corp. and Ford Motor Co. are offering packages to dealers who want to offer oil changes that would take about a half hour and cost around $20.

Typically, quick oil changes involve a new oil filter, oil, spot lubrication on doors and some suspension joints, filling brake and windshield washer reservoirs and checking tire pressure.

Dozens of companies have sprung up nationwide offering the service in drive-through bays, and the idea is a topic among members of the National Automobile Dealers Association, which is holding its annual convention here.

The idea is to get consumers into the dealership's service area on a routine, non-warranty basis. That can result in increased service or parts business and quickly become the bread and butter for the dealers.

Many of the nation's 20,000-plus auto dealers, about half of whom are not making money, rely on profits generated by their parts and service departments to keep them financially afloat. The setup also would profit the automakers' in-house parts operations.

But money stands in the way for Tony Fuentes, general manager of Don Thornton Ford of Tulsa, Okla. He said the Ford program, which includes a lift, close-at-hand tools and parts, would cost him $24,000.

''In the past,'' he said, ''people would not think of a dealership for this stuff. You went to dealerships for warranty work.

''Once the warranty expires, bang, they go off to an independent'' mechanic.

Wayne Phillips, a consultant for the NADA, said he's found that dealers are interested in the operations, but cautious about them.

''In some areas, it can be a very profitable thing,'' he said, especially if the dealership is on a busy street.

Ford was the first to offer quick oil changes nationwide. Five dealer in Chicago had it in June 1988. That number grew to 25 by October 1988. General Motors Corp. started a pilot program at about 100 dealerships in and around four manor cities in 1989 and Chrysler Corp. also has some dealers offering the service.

Dealers are approaching the offers in a variety of methods.

Colonial Dodge in Rockville, Md., offers an oil change and quick lubrication for $9.95.

''We want more traffic in the back'' of the service department, where more expensive work is performed, said Al Briggs, general manager of the dealership.

George Hornbeck III, owner of Hornbeck Chevrolet-Cadillac in Carbondale, Pa., isn't interested in the operations.

''I kind of think I'm not going to go that way,'' he said. ''I'm not ready to put that money out for something I already do.''