Exxon Corp., with 6 million oil credit-card customers, announced yesterday major changes in its revolving-charge-account policies aimed at discouraging the use of credit.

Beginning Aug. 1, small single purchases of less than $40 must be paid off within 30 days. Larger purchases may be charged on revolving credit, but minimum payments on those accounts are increased by one-half, from $20 to $30. Under the new rules, revolving credit for gasoline purchases would be virtually eliminated.

In addition, credit limits for revolving accounts will be reduced from $500 to $300, and finance charges will be increased in some states.

The changes, announced yesterday afternoon, marked the second attempt by a major oil company to reduce credit. Last month Mobil Corp. announced it would stop issuing credit cards and double minimum monthly payments from $10 to $20.

The changes announced by Exxon will affect only customers who use credit cards for revolving credit, not the majority of customers who pay their accounts in full within 30 days, an Exxon spokesman said.

The company said its actions were "consistent with the intent of President Carter's anti-inflation program which attempts to restrain the growth of consumer credit."

If a customer chooses not to add to his or her revolving balance after Aug. 1, he or she may pay it off by using existing payment terms, according to the company.

Changes in finance charges will affect primarily rates applied to balances of more than $500, according to Exxon. Finance charge rates will be either 1.5 percent a month or the maximum allowed by state law.

Only in five states -- Connecticut, Minnesota, South Dakota, Wisconsin and Hawaii -- will balances below $500 be affected. In eight states -- Arkansas, Maryland, Massachusetts, New York, Pennsylvania, Texas, Vermont and Washington -- the change in rates won't have an impact.

Exxon also announced that it will adopt an average - daily - balance method of computing finance charges, as have major retailers such as Sears.

In other developments yesterday, the government acted to help credit unions to compete with commercial banks and thrift institutions such as savings and loan associations by increasing the maximum amount of interest credit unions may pay on deposits.

The National Credit Union Administration, which regulates the country's credit unions, increased interest ceilings on share accounts and retirement accounts to 9.5 percent effective Monday.

The new ceilings on share certificates apply to certificates ranging from 90 days to six years. Current maximum interest available from such deposits is 8.55 percent.

The ceilings may go as high as 12 percent if average yields on 30-month Treasury bills exceeds 9.5 percent.

The NCUA also announced changes in maximum interest payable on six-month savings certificates beginning next week. Under the new ceiling, credit union members will be able to earn one-quarter of a percentage point more than the prevailing discount rate on Treasury bills.

Earlier this week the Depository Institutions Deregulation Committee, created by Congress, allowed banks and thrift institutions to pay a quarter of a percentage point more on six-month money market certificates when Treasury bill rates rise above 8.75 percent.

P.A. Mack, the NCUA's vice chairman, said yesterday's action was aimed at protecting credit unions from a competitive disadvantage in attracting savers. Like credit unions, banks and savings and loan associations have lost deposits as savers shopped for better deals, buying Treasury bills and other investments.