At the Old Dominion Exxon, 4035 Old Dominion Dr. in Arlington, the credit-card customer pays 4 cents a gallon more than the cash customer.

At the Van Ness Texaco, 4225 Connecticut Ave. in Northwest Washington, the nighttime price for gasoline is 10 cents a gallon more than during the day.

And at the Four Corners Amoco, 112 University Blvd. in Silver Spring, the full-service charge for regular unleaded gas is 34 cents a gallon more than the self-service charge. Full-service regular leaded is 28 cents a gallon more, and full-service premium is 11 cents a gallon more.

Those are three examples of the different marketing practices that confront the Washington area motorist in search of a full tank of gas. Such variations always have been important to motorists, even with gas prices dropping. But the recent round of oil company price increases has given motorists new reasons to check station pricing rules before adding gas to the car tank.

More incentive is just around the corner, with the implementation of the additional 5-cent-a-gallon federal gasoline tax on Friday. Many dealers are expected to pass that along to motorists, although the amount of the increase will vary from station to station, depending on competition.

Here are three steps drivers can follow to keep gas bills to a minimum in today's volatile fuel market:

Read all the signs--the big ones and the small handwritten notes.

Alice Goldman was on her way to work when she stopped shortly after daybreak one morning recently to buy gas at the Van Ness Texaco. "There was a sign saying unleaded cost $1.06 a gallon, so I filled the tank," Goldman said.

But when she went to pay for the gas, she learned she would have to pay $1.16 a gallon because the station had a night rate still in effect at the time of her purchase.

Consumer officials who investigated Goldman's complaint found a small, handwritten note on the pumps about the night and day rates and said that the note satisfied legal notification requirements.

But Goldman is still annoyed by the experience. "I felt like it was false advertising because there was a huge sign, like a movie marquee, saying $1.06 for unleaded gasoline and there was nothing on it at all to indicate that that was a daytime price."

Koo Yuen, the station's operator, said the night differential represents the additional costs of keeping the station open late hours and the notice to customers is adequate.

* Look for the postings on all grades of gas, not just regular leaded.

"If you see a sign that says leaded regular is 98.9 cents a gallon, don't assume that the unleaded regular and the premium gasoline also are low," warned Tom Crosby, a spokesman for the American Automobile Association's Potomac Division.

One recent AAA survey found a dealer selling self-service regular leaded for 98.9 cents a gallon and self-service unleaded for $1.209 a gallon--a 22-cent difference. That was three times the average difference of 6.9 cents a gallon found in the survey.

* Watch out for excessive differences in the station's price for full service and self service.

On the average, area motorists can save 23.3 cents a gallon by pumping their own fuel, Crosby said. But some stations charge as much as 40 cents a gallon more for full-service gas than for self-service, he said.

The random sampling of 111 area gas stations by the AAA also revealed these price trends:

* One-third of the stations surveyed charged less than $1 a gallon for regular leaded gas. The lowest prices were found at two stations in Prince George's County, where self-serve cash customers were charged 96.6 cents a gallon. The highest price was at a Northern Virginia station where full-service premium unleaded was $1.629 a gallon.

* Suburban Maryland stations generally charge the lowest prices in the metropolitan area, with all grades of gas averaging $1.236 a gallon. There was a virtual tie between Virginia, with an average of $1.260 a gallon and the District, with an average of $1.267.

Although the AAA normally urges motorists to patronize the lower-priced pumps because of the potential savings on gas bills, the car club has begun to suggest occasional purchases at full-service stations.

"It's an excellent idea for motorists to patronize a full-service station at least every third or fourth trip to the pumps," said Robert Livingstone, manager of the local club's approved auto-repair program.

"The per-gallon savings that comes from only using self-serve pumps doesn't really amount to much if neglect of car maintenance results in wasted fuel or perhaps even a major breakdown. Periodic full-service stops for under-the-hood checks of things like oil, radiator and battery fluid levels and the condition of hoses, belts and tires could prevent serious mechanical problems."