Eight members of the Senate Foreign Relations Committee yesterday signed a letter to President Reagan expressing "serious concerns" about the possible sale to Saudi Arabia of sophisticated additional equipment for the 60 F15 jet fighters it has ordered from the United States. e

The equipment -- extra-large fuel tanks and bomb racks that would increase the range and firepower of the jets -- is a matter of great concern to Israel, which fears that the planes could be used against Israeli territory in some future worsening of the Arab-Israeli conflict.

The letter, delivered to the White House last night, was prepared as Israeli Foreign Minister Yitzhak Shamir arrived here for several days of talks that will include meetings today with Secretary of State Alexander M. Haig Jr. and Defense Secretary Caspar W. Weinberger. Shamir also will confer with Reagan next Tuesday.

In recent contacts between Haig and representatives of Prime Minister Menachem Begin, the Israelis are known to have signaled that they might drop their opposition to the sale if Washington compensates them with additional military equipment on favorable terms that would not put heavy new burdens on Israel's strained financial situation.

Although talks are still in a preliminary state, administration policy-makers are known to be encouraged that a deal can be worked out along these general lines. The administration is anxious to sell the equipment to Saudi Arabia, which is America's principal oil supplier and has threatened to make satisfaction of its request at a test of the special U.S.-Saudi relationship.

However, the administration also wants to avoid a fight with Israel's supporters in Congress, where the sale can be blocked if both houses vote against it. For that reason, forwarding of the letter at the time of Shamir's visit appeared to be aimed at putting additional pressure on the administration to come to terms with the Israelis.

The administration is known to believe that, in a showdown, the new Republican majority in the Senate would back Reagan on the sale. In what appeared to be an underscoring of that assessment, the signatures on yesterday's letter did not represent a majority of the 17-member Foreign Relations Committee.

Those signing were seven Democrats -- Joseph R. Biden (Del.), John Glenn (Ohio), Christopher Dodd (Conn.), Alan Cranston (Calif.), Paul Sarbanes (Md.), Paul Tsongas (Mass.) and Claiborne Pell (R.I.) -- and one Republican, Larry Pressler (S.D.).

However, Senate sources contended that a majority on the committee is presently opposed or leaning against the sale. The sources said one other Democratic member, Edward Zorinsky (Neb.), is so strongly opposed to the sale that he refused to sign on grounds the letter was not strong enough.

The sources also said that three Republican members -- Charles McC. Mathias (Md.), Rudy Boschwitz (Minn.) and Nancy Kassebaum (Kan.) -- declined to add their signatures at this time but made clear they have strong concerns and reservations about the sale.

In addition, Sen. Edward M. Kennedy (D-Mass.), who is not a committee member, said yesterday he is "strongly opposed" to selling the equipment to the Saudis.

Sale of the F15s, whose delivery is scheduled to begin in 1982, was approved in 1978 after a stormy battle in the Senate. At the time, then-defense secretary Harold Brown met objections from pro-Israeli senators with the assurance that the jets would be purely for defensive purposes and would not have the additional equipment that would enable them to be used against Israel.