NEW YORK, JAN. 27 -- Texaco Inc. said today that if its proposed bankruptcy settlement plan is approved, it expects to report an unprecedented $4.5 billion loss for 1987.

The loss, which would be the largest in U.S. corporate history, reflects both the cost of Texaco's proposed $3 billion legal settlement with Pennzoil Co. and writeoffs associated with a corporate restructuring planned by Texaco once it emerges from bankruptcy.

The estimated loss was reported by Texaco in an amended disclosure statement filed in federal bankruptcy court today. Judge Howard Schwartzberg, who oversees the company's bankruptcy proceedings, opened hearings today aimed at approving a final disclosure statement about the bankruptcy settlement. The statement will be sent to Texaco's shareholders in connection with a vote on the proposed reorganization plan.

Texaco reached a proposed bankruptcy settlement last December with Pennzoil and committees representing its shareholders and creditors. The settlement is designed to end Texaco's $10.3 billion legal battle with Pennzoil, which arose over a disputed 1984 merger between Texaco and Getty Oil Co.

Separately, it was disclosed today that the Securities and Exchange Commission has expressed concern to Texaco that its disclosure statement does not adequately explain why the company released other parties in the Getty deal -- including law firms and investment banks -- from all legal liability as part of the proposed settlement with Pennzoil.

The SEC said it would review the amended statement to see if its concerns have been addressed in the latest draft and then report back when the disclosure hearings resume Friday. But Richard Kirby, the SEC assistant general counsel who supervises the commission's bankruptcy program, said tonight that his initial review of the amended statement suggests that Texaco may yet have to answer more questions for its shareholders.

Texaco, Pennzoil and other parties involved in the settlement negotiations agreed to terminate more than a dozen lawsuits filed against Texaco's directors and others involved in the Getty Oil merger. In doing so, Texaco sacrificed its ability to recover some of the $3 billion it is paying to Pennzoil by suing other parties in the Getty deal.

Critics have accused Texaco's management of protecting itself and its directors at the expense of shareholders by acquiescing to the broad legal indemnities. In its original disclosure statement, Texaco explained its willingness to end the derivative suits by saying it would "permit management to focus on business operations and methods to enhance the value of Texaco stock."

Kirby said tonight that the SEC wants Texaco to fully explain, among other things, whether the company investigated the possibility of suing other parties in the Getty deal to recover part of its settlement costs.

As earlier reported, Texaco did retain a retired federal judge to explore the issue of possible lawsuits against other parties while the Pennzoil case was active, but the judge, Joseph Morris, never reported his findings to the Texaco board, according to sources.

The amended earnings estimates filed by Texaco today show an expected $470 million profit during 1987 from Texaco's worldwide oil and gas operations. But that profit is overwhelmed by about $4.9 billion in writedowns arising from the Pennzoil deal and the proposed restructuring.

Texaco said earlier this month that its restructuring likely would involve the sale of partial interests in refineries and other assets and the formation of joint ventures with outside parties. But the company has not yet identified its prospective partners or provided details about the deals under discussion.

The details and financial impact of the planned restructuring are of particular interest to Texaco shareholder Carl C. Icahn, chairman of Trans World Airlines Inc. Icahn has publicly encouraged efforts by Texaco's management to raise the price of its stock through a restructuring, but he opposes the pending bankruptcy settlement because it does not remove the company's takeover defenses.

Texaco's financial problems are not limited to the costs of its deal with Pennzoil and the restructuring now under review. The company was informed earlier this month by the Internal Revenue Service that it may owe $6.5 billion in back federal taxes.

Texaco said that a substantial portion of the IRS's claim derives from a novel theory about Texaco's sale of Saudi Arabian crude oil between 1979 and 1981. The company said it would dispute that and other IRS claims.

A representative of the IRS, appearing at today's hearing, said the service would not press its tax claims against Texaco as part of the bankruptcy reorganization but would litigate the claims through customary tax proceedings. Such litigation would likely take years to finally resolve.