Washington area gasoline prices have continued their long decline by dropping 2.1 cents a gallon over the past five weeks, according to the Potomac division of the American Automobile Association.

The average price for all grades of gasoline was $1.252 a gallon, 20 cents a gallon lower than the price approximately two years ago and 8.3 cents below the cost of gasoline as recently as Christmas. In fact, prices as of Wednesday were the lowest since March 1980, the AAA reported.

The survey also found that more than two dozen of the stations surveyed (almost one-third) charged less than $1 a gallon for regular leaded gasoline, with two stations in Prince George's County charging 96.6 cents a gallon for customers who pump their own gasoline and pay cash.

Diesel prices dropped more sharply than gasoline prices, falling 5.4 cents in five weeks to an average price of $1.256 a gallon.

Most stations had changed their pump prices during the past two weeks, and more than half said they expect additional changes before the federal gasoline tax increases by 5 cents a gallon on April 1.

The organization reported that local travel has increased 9 percent in the first two months of 1983 compared with a year ago. Increasing demand as summer travel picks up may help hold pump prices down.

According to the survey, the lowest average price per gallon was $1.236 in suburban Maryland compared with $1.260 in Northern Virginia and $1.267 in D.C. The average savings realized by a customer pumping his or her own gas was 23.3 cents a gallon. The AAA also found that motorists can save as much as 60 cents a gallon by shopping around for the best buys.

Robert Livingstone, manager of AAA Potomac's Approve Auto Repair Program, cautioned against relying entirely on self-service. "It's an excellent idea for motorists to patronize a full-service station at least every third or fourth trip to the pumps," Livingstone said. Such visits allow for useful preventive maintenance, he said.