Central Intelligence Agency Director Stansfield Turner said yesterday that U.S. ambassadors are better informed than ever about CIA operations abroad, but he insisted that they still don't need to know all the details.

Testifying before the House Intelligence oversight subcommittee, Turner said that in most cases, the CIA will withhold the identity of its sources and the names of its agents, especially if they are foreign officials with whom an ambassador deals on a regular basis.

"It's not because we don't trust the ambassador," the CIA director declared. But under what he wryly called "Turner's Law," he said that the danger of leaks increases in "geometric proportion" to the number of people who know a particular secret.

Especially hazardous, Turner indicated, was the prospect of an ambassador making some giveaway slip of recognition on greeting a foreign minister in a receiving line when the ambassador knows the minister to be a CIA "asset."

"We've had instances of this," the CIA chief told the subcommittee headed by Rep. Les Aspin (D-Wis).

"In order to protect information, you must sometimes dissimulate," Turner said, but that, he emphasized, "takes training."

Turner made his remarks under questioning by Aspin about the extent of CIA compliance with a directive from President Carter making the American ambassador the person in charge of all U.S. operations in a given country. The Wisconsin congressman cited reports that the CIA may have been hedging on that directive.

"I say to you, sir, that that's balder-dash," Turner protested. Under an agreement he signed with Secretary of State Cyrus R. Vance last spring, he said, CIA station chiefs abroad have become more, not less, candid with their ambassadors. While some information might be held back from an ambassador, Turner said:

"It doesn't mean he's not in charge. It means he doesn't need all the details to be in charge."

According to the CIA director, there is no rule dictating that a U.S. ambassador must be told everything the CIA is doing - and no rule against it either.

"That's a matter for negotiation between the ambassador and the chief of station," Turner told the subcommittee. "The chief of station either volunteers or he doesn't volunteer . . . the ambassador either asks or be doesn't ask."

Disputes over how much information to share are supposed to be settled by Vance and Turner, but the CIA director said none has reached that stage. Only one or two ambassadors abroad have voiced any complaints, he added.

"Some ambassadors are very possessive," Turner said in an unusually detailed accounting of the persistent rivalries within U.S. embassies around the world. Occasionally, he said, an ambassador will insist on exclusive control over the political intelligence assessments to be sent back to WASHINGTON. The CIA has refused to go along, the subcommittee was told.

"I can't saccept that," Turner testified.He said he wants, and demands, separate assessments from his CIA missions.

Most of the morning-long hearing was devoted to Turner's promulgation last Nov. 30 of new rules governing the CIA's relationships with U.S. news media. They prohibit the CIA from approaching journalists accredited by a U.S. outlet "for the purpose of conducting intelligence activities.

Turner said the regulations forbid the CIA from asking "even a willing journalist to seek out or furnish particular information or assistance."

"A U.S. media representative, under these regulations, could quite legally work for the KGB, but not for me." The CIA director told Rep. John M. Ashbrook (R-Ohio) at one point.

Turner acknowledged that the rules do not cover foreign-owned news services, such as Reuter, with outlets here or organizations such as the former U.S. Information Service or Radio Free Europe. In addition, the rules govern only the CIA and not other U.S. intelligence agencies.

Critics have charged that the rules are loaded with loopholes, including one permitting Turner to make any exception he deems necessary, but the CIA director said he has yet to use the "escape clause."

Rep. Wyche Fowler Jr. (D-Ga.) said he thought Turner had done "an extraordinary job" in protecting American journalists from temptation, especially the hungrier ones.

"As we all know, except for Washington," Fowler said, "most journalists are hungry. Up here, they loosen their belts after lunch."