ust when you might have thought Paul Newman's salad days were behind him, the 57-year-old actor has turned to salad dressing.

Hundreds of celebrities have lent their faces and names to other people's products over the decades, but Newman is using his famous blue-eyed visage to sell his very own mixture of oil and vinegar, beginning a third career that could make him a star in the kitchen.

Since it first came on the market three weeks ago, at Gold's Delicatessen here in Westport, where Newman makes his home, and at Stew Leonard's Dairy in neighboring Norwalk, shoppers have bought an estimated 10,000 bottles of "Newman's Own," an all-natural oil-and-vinegar salad dressing. Hand-lettered store signs announce "Paul Newman's own recipe," and, at Stew Leonard's, "Buy two, get one head of lettuce free."

"They're flying off the shelves," exults David Kalman, a partner in Advantage Food Marketing of Westport, which is overseeing distribution of the salad dressing. Kalman says it is also available in King's Supermarkets, a New Jersey chain, and will be in New York's Shopwell and Food Emporium stores in two weeks. After that, distribution of the mild-tasting dressing, which sells for $1.19 to $1.39 a bottle, will be expanded to cover the Northeast, from Washington to Boston.

A spokesperson for Newman, who said that the actor was unavailable for comment because of commitments in his second career of auto racing, emphasized that Newman had entered the food field "as a lark" and that he plans to donate his profits to charity, "probably something in the drug rehabilitation field" (Newman's son died of an overdose).

In keeping with the light-hearted nature of the venture, the dressing's parchment-colored label identifies "Newman's Own" as (in very small type) "Appellation Newman Controle'e" and (in even smaller type) "l'etoile du vinaigre et de l'huile -- l'huile et le vinaigre des etoiles," which is fractured French for "star of vinegar and oil--oil and vinegar of the stars" (in unfractured French the word "vedette" connotes star, as in celebrity).

There was every indication today that customers were responding to the dressing in the proper spirit.

"Are you serious?" asked Stella Hartel, who had just put two bottles of "Newman's Own" in her shopping cart. She giggled when asked what she thought when she first saw the bottle with the movie star's smiling face etched on the label and "au naturel" written over his head. "You really want to know? You can't print it," she said. "It's very funny. I see him all the time on the street and I thought, you know, I'd help him out."

Another buyer, Kathy Clarke, said she lives "about four houses" from Newman and was buying the dressing "as a fun party gift. I'm giving it wrapped like a bottle of wine with a head of lettuce." Then Clarke, who was wearing rose-colored glasses, added in a tone of confidential glee, "It's really a riot, don't you think?"

Sheena Berg, with two children in tow, was making her second purchase of "Newman's Own." "It really has a very good flavor," Berg said. "It tastes like a homemade vinaigrette. Also it's a novelty. We had friends over the other night. Everyone was shocked that Paul Newman was doing something as tacky as putting his face on something like this."

Tacky or not, the venture began just after Christmas 1980. Following his custom of many years, Newman and author friend A. E. Hotchner ("Sophia: Living and Loving") had brewed and bottled a large batch of the actor's formula of olive oil, soybean oil, red wine vinegar, water, lemon juice, spices, salt, onion and garlic to give to friends and neighbors as holiday gifts. Then, according to a short note written by the actor, which appears on the back of the label, "the acclaim was deafening, the repeat orders staggering . . . Enough, I said! Let's go public! I'm out of the basement and onto the shelf."

The leap from basement to shelf required 18 months, says Julius Gold, proprietor of Gold's Delicatessen, where "Newman's Own" is displayed alongside such other local delicacies as "the original preppy cookie" (Southbury, Conn.) and "Dr. Mike's Ice Cream" (Bethel, Conn.). First there were a number of tasting sessions, "usually around Hotchner's table," while the manufacturer -- Ken's Foods in Framingham, Mass. -- sought to duplicate the Newman dressing for mass production. Then there was the label, designed by Ellen Posey, artist, photographer, and wife of race-car driver Sam Posey, a colleague of Newman's on the driving circuit since 1978.

"When Paul and I first talked about it, he wanted to establish sort of a wine-label look," Ellen Posey said. "He wanted to put it in a dark green bottle with a cork. But that was too expensive, so in the end there were many compromises."

Posey says that she even had to compromise regarding the sketch of Newman, framed with entwined vegetables, which she drew for the front of the bottle. "I favored a sterner looking Paul, but the overall consensus was for a happy Newman."

A laughing Newman, one suspects--laughing all the way to his charities.