Shell Oil Co. has begun selling gasoline here by the liter instead of the gallon -- a change that probably will become standard practice in most of the Washington area's 1,000 service stations during the next two years.

While Shells publicized its move as the "inevitable" wave of the future, customer reaction was mixed yesterday at a Falls Church Shell station that made the switch last week.

"I think it's stupid, what's the reason?" asked one woman customer as she bought gasoline. "Why can't we use the old scale?"

However, Howard Holbrook bought 30.6 liters of gasoline for $10.02 at the self-serve pump and said he liked the new system better because he could see the exact price that he had to pay registered on the pump.

The station's pumps had signs explaining that 3.785 lieters equal a gallon and giving the price in both liters and gallons. For the record, a litter is slightly bigger than a quart -- 1.057 quarts, to be exact -- or put another way, a quart equals .946 liter.

In another development Exxon, the area's second largest retailer, said that because of rising crude oil prices it would raise wholesale gasoline prices today by four cents a gallon. The increase is expected to be passed on to retail customers.

A Shell spokesman said the company is changing to liters here and across the country because most of its pumps compute prices only up to 99.9 cents and the resulting half-gallon pricing has confused customers.

With a gallon of gasoline now costing about $1.20 here, virtually all area stations are facing this problem. If they convert first to pumps that will register more than $1 a gallon, they may later have to adjust their pumps again, as part of the national switch to metric measurement.

Shell plans to convert all its 165 Washington-Baltimore area stations by this summer and the rest of its stations across the country by the end of this year or early next.

Exxon, with 450 stations here, plans to convert by 1982. Amoco, another big Washington distributor, plans to convert but is not sure when. "We're confident this is the way the industry is going to go," an Amoco spokesman said yesterday.

Some area consumer officials expressed concern that the change to liter sales of gasoline will sow confusion among comparison shoppers.

"Certainly the process of change is going to be very difficult for consumers," said Barbara Gregg, director of Montgomery County's consumer affairs office. "I think people will be confused, and when they are more apt . . . to be taken advantage of."

Shell has taken elaborate precautions to keep people from being confused. Each customer at the Falls Church station yesterday received a leaflet explaining the metric system and containing tables for converting liters into gallons.

Roland Jones, the Falls Church dealer, had posted a big sign that was visible from the highway: "Shell Regular self-serve 31.9 (per) Liter." The pumps also carried signs explaining the differences in measurement.

Each pump had signs explaining that 3.785 liters equaled one gallon and giving the price per liter and the equivalent price per gallon.

For example, regular gasoline at the full-serve pump cost 32.0 cents per liter or 121.4 cents per gallon, according to the sign.

Fortunately for Jones, the price of gasoline has not gone up since he posted the signs last week. It went up 3 cents a gallon just before he posted them.

Despite all this, there was confusion.

"I just don't know anything about it," said Terry Barr as she asked for $3 worth of gasoline. "What is a liter?"

While the move to a metrics in gasoline sales is being hastened by rising prices and the limitations of gas pump dials. It is only part of a national move toward metric measurements for other reasons.

According to a spokesman for the U.S. Metric Board, because of the "increasingly global marketprice," Congress has encouraged American Industry to produce goods measured in a centimeters rather than inches and grams rather than ounces.

In other words, if American goods are to be successfully sold and serviced abroad, it would help if they are built and measured by the metric specification used by virtually all other countries.

Gregg and other consumer advocates are worried that in the confusion of the changeover, producers can sell smaller packages of goods for the same amount as they used to sell larger packages.

Even our eyesight may soon be checked over a metric distance and those with perfect eyesight pronounced to have "4-4" rather than "20-20" vision. The old 20-20 means seeing a certain-sized letter at 20 feet, and the new 4-4 means seeing a smaller sized letter at 4 meters -- a little more than 12 feet.