The biggest dip in energy costs in nearly a year partially offset higher food and automobile prices in September as wholesale prices rose a moderate 0.3 percent, the government reported yesterday.

The increase in the Labor Department's producer price index left wholesale inflation for the first nine months of 1987 at an annual rate of 3.6 percent.

The September rise in prices, one stop short of the retail level, followed a month in which prices remained level. Wholesale prices had risen 0.2 percent in July.

Analysts generally said the report buttressed contentions that inflation remains relatively tame, despite apprehension in financial markets, which has pushed up interest rates in recent days.

Still, if September's swings in energy and food prices are discounted, the index for all other goods rose a fairly sharp 0.6 percent.

"The report is not quite as good as it appears on the surface," said Robert Ortner, undersecretary of Commerce for economic affairs. "The moderate gain was achieved with a drop in oil prices."

Energy costs, up in seven of the eight previous months, dropped 3.7 percent in September, paced by a 6.4 percent tumble in gasoline prices and an 11.4 percent fall in home heating oil.

It was the biggest decline in energy prices since an identical 3.7 percent drop in October 1986.

Food prices, meanwhile, moved up 1.1 percent, all but erasing a 1.3 decline in August. Fruits, vegetables, pork, beef and eggs were all up. Fish and poultry prices fell.

In other economic statistics released yesterday:

The Federal Reserve reported output at the nation's factories, mines and utilities edged up a small 0.2 percent in September, the eighth consecutive month without a setback.

The Commerce Department said business sales shot up 1.3 percent in August while inventories rose 0.2 percent, just half the pace of the previous month.

The increase in wholesale prices was in the range considered modest by analysts and held few surprises.

"It was very much in line with expectations," said David Jones, senior economist for Aubrey G. Lanston & Co. in New York. "What we had was offsetting moves in the two most volatile commodities. Food was up and energy was down."

Passenger car prices were up a hefty 3.6 percent in September, following a 1.8 percent decline in August. The increase reflected smaller incentives offered by manufacturers to dealers in September for new-year models.

The September rate alone was equivalent to an annual, compounded rate of inflation of 3.7 percent.

Dirk Van Dongen, president of the National Association of Wholesaler-Distributors, said "producer price activity doesn't occur in a vacuum. As the stubbornness of the trade deficit reflects itself in rising interest rates, I'd look ahead with some degree of caution to possible producer price inflation in the months ahead."

Wholesale prices have been rising at a slower pace this year than prices at the retail level. The companion consumer price index rose through August at an annual rate of 5 percent.

The consumer price figures -- which more directly reflect inflation from rising import prices -- for September will be released next week.

"On balance, the {producer price index} is not growing very rapidly. It does not show any major acceleration of inflation," said Lawrence Chimerine, president of Wharton Econometrics, a private economic forecasting service. "I'm not concerned about this report."

The September increase left the producer price index at 296.7, meaning that goods which cost $10 in a 1967 base period cost $29.67 in September.

That compares with a $28.75 price tag for the same group of products in September 1986.

The index itself, unlike other figures in the monthly report, is not adjusted for seasonal fluctuations.

The Labor Department gave these other specifics on September wholesale price activity:

Egg prices surged 21.2 percent. Beef and veal were up 1.8 percent. Poultry was down 4 percent.

Natural gas costs rose 0.5 percent after a 2.6 percent fall in August.

Machinery and other capital equipment jumped 0.7 percent after a 0.2 percent increase in August.