Mobil Corp., the country's second-largest petroleum concern, today became the latest entrant in the increasingly expensive war to take over Conoco Inc., the nation's ninth-biggest oil company.

Mobil weighed in with a record $7.74 billion stock and cash takeover bid for Conoco.

Conoco is already being pursued on a "friendly" basis -- that is with the approval of Conoco's board of directors -- by the big chemical company E.I. duPont de Nemours & Co. and on an unfriendly basis by Seagram Co., the Canadian-based distiller.

If anything, the Mobil bid makes it more difficult for a Conoco shareholder to decide what to do with a stock that was selling for less than $50 several months ago and closed on the New York Stock Exchange today at $87.50.

"This is not the last bid for Conoco," said Robert Morris, an oil analyst at the big brokerage firm Drexel Burnham Lambert Inc. "The Mobil bid forces somebody else to make a move."

Mobil said today it will pay $90 a share in cash for 43.5 million of approximately 86 million outstanding shares of Conoco and will exchange Mobil securities worth roughly $90 a share for the remaining Conoco stock. The cash opart of the Mobil bid totals $3.915 billion and the securities portion about $3.82 billion.

Du Pont, the biggest chemical producer in the United States, has offered $7.4 billion in cash and securities -- $95 a share for up to 34.44 million shares (40 percent of Conoco stock) and 1.7 shares of its own stock for each of the remaining shares.

Seagram said it wants to acquire 51 percent ownership in Conoco and will pay $85 a share in cash for 44.35 million shares of Conoco, a total outlay of $3..77 billion.

Mobil has a $6 billion credit line with its banks, while Du Pont has a loan commitment of $4 billion. Seagram has $3.8 billion in loans and cash and the ability to borrow more if it needs to and wants to.

Conoco's oil and gas reserves, as well as its extensive coat reserves, probably are worth as much as $200 a share, so the bidding for the oil giant could go much higher. Du Pont wants Conoco, at least in part, to give it an assured source of oil for its vast petrochemical operations. Mobil wants Conoco's natural resource holdings while Seagram, controlled by the Bronfman family, also is interested in buying a company with natural resource reserves.

While the Mobil offer seems richest, both Mobil and Du Pont have said their offers are null unless they receive at least 44.35 million shares of Conoco. Seagram, on the other hand, has committed itself to buying every share it receives up to 44.35 million.

"With three firms in the bidding, a stockholder runs the risk of being left high and dry if he tenders [offers to sell] his shares to Mobil or Du Pont and one or the other pulls out," said a Wall Street trader with a significant holding of Conoco stock. "On the other hand, if you make the sure well to Seagram, you run the risk of losing $10 a share or more."

He said the Mobil bid was designed to force Du Pont and Seagram either to make new offers or pull out of the bidding. Texaco Inc., the country's third-largest oil company, has a newly arranged $5.5 billion credit line of its own and is rumored to be interested in Conoco as well. Texaco had talks with Conoco before Conoco turned to Du Pont to save it from the Seagram offer.

Even though a merger of Mobil and Conoco would be among two giant oil companies -- Mobil's sales last year were $59.5 billion, second only to Exxon's $103.1 billion, and Conoco's were $18.3 billion -- Mobil said that using guidelines developed by the Johnson administration in 1968, it does not think such a merger would violate antitrust laws. Antitrust experts in Washington and New York said Mobil might be right.

Du Pont, with an annual sales last year of $13.7 billion, overlaps with Conoco only in the production of petrochemicals, while Seagram is mainly a producer and seller of wines and spirits with no oil properties in the United States. Seagram is making its run at Conoco in part with the $2.3 billion it reaped last year from the sale of its U.S. oil and gas holdings to Sun Oil Co.

Conoco Chairman Ralph Bailey said that the Conoco board would review the Mobil offer next week but said the proposed merger "raises substantial antitrust and other public policy issues which will require intensive examination and which make the acquisition of Conoco by Mobil subject to serious question."

Both Du Pont and Seagram declined to comment on the Mobil offer, but both said they think their proposals are the best for Conoco shareholders.

Bailey, late in the day, took a swipe at the Seagram offer, Too. He said that the government of Dubai threatened to cancel Conoco oil leases if the company is controlled by Seagram because Dubai had bad experiences with a Seagram subsidiary in the past. Conoco's oil-producing subsidiary in that Middle East kingdom had revenues of $1.2 billion last year.