A $5.5 billion bank loan, the largest international bank credit ever organized, is being put together for Texaco Inc., according to banking sources in Europe. Texaco, the third-largest U.S. petroleum firm, is reported to be on the verge of making a takeover bid for another U.S. oil company.

Separately, Conoco Inc. aid it has boosted its lines of credit at banks to $3 billion, and some financial community sources, said Conoco may fear its planned merger into E.L. du Pont de Nemours & Co. is in trouble and is raising a defense fund in case it has to fight off an unwanted suitor, Associated Press reported.

[Wall Street analysts speculated that Texaco may be planning to make a bid for Cites Service, the 20th-largest U.S. oil firm. Earlier, there had been speculation about a Texaco-Conoco combination. In a statement, Texaco said only: "As a matter of policy, we do not comment on such reports."]

Cities Service has denied it is engaged in merger discussions. A Conoco spokesman said the company would have no comment. However, analysts said that even if Texaco is considering a bid for either company, the target company would not yet have to know that a takeover was even in prospect.

Texaco's loan dwarfs the previous $4 billion record laon, a two-year bankers acceptance facility just completed for Pemex, the Mexican state oil company.

The eight-year laon for Texaco is being organized by Chase Manhattan Bank. Because of the size of the operation, Chase has to seek the participation of major banks around the world. Interest on the loan will be more than 18 percent at current rates.

At the same time, Chase is syndicating another $3 billion loan for Du Pont to finace the cash offer it has made in its bid for Conoco, a deal estimated at about $7 billion.

Seagram, the Canadian distilling giant, set a record last year when it arranged a $3 billion syndicated loan. The proceeds of that loan were clearly intended as a kitty for a takeover bid, and some banks refused to participate without assurances that the bid would be a friendly one. The banks did not want to be embarrassed by financing a bid that might be regarded as unfriendly for one of their own clients.

Seagram has since made two unsuccessful attempts to buy U.S. companies -- St. Joe Minerals, which preferred to merge into Fluor, and Conoco, for which Du Pont earlier this week offered more money.

The terms on the Texaco loan give participating banks the choice for the first five years of accepting the prime rate, as quoted by Chase, or 3 percentage points over the London interbank rate, currently 18 3/16 percent.

Terms for the final three years call for the rates rise to a quarter of a percentage point over the Chase-quoted prime rate or half a point over the six-month London interbank rate. In addition, banks are offered a quarter-percent commitment fee. Only the best-rated industrial countries, such as France, have borrowed in the Euro-market at lower rates.

Seagram has offered $73 a share for 40 percent of Conoco and made an until now-secret offer of $85 a share.

Despite its traditional reluctance to comment, Texaco announced earlier this week, "in response to a request from Conoco," that it had held "informal discussions" for an "all-cash merger" with the company but that Texaco had made "no merger offer."

The Conoco credit line replaces a $1 billion line. The new $3 billion credit was paid by the company to be for general corporate purposes. But Constantine Fliakos, an analyst at Merrill Lynch, Pierce, Fenner & Smith, said "it sounds to me like too much" for that use. He said Conoco might be considering bidding for its own shares if that becomes necessary.

[Conoco was most active on the New York Stock Exchange today, up $1 a share to $77.37 1/2 on a volume of 824,300 shares. Texaco was third on the active list, up 12 1/2 cents to $35.12 1/2.]

[With speculation that it was the destination of a least part of Texaco's projected borrowing, Texaco Canada Inc. issued a statement that Texaco Inc. was not buying any of the shares of its partly owned Canadian subsidiary. At the end of 1980, Texaco owned 89.7 percent of Texaco Canada.]

[With the new credit, Texaco's takeover power would be impressive. At the end of the first quarter, the company had $3.6 billion in cash, and at the end of 1980 it had $1 billion in unused credit. It was unclear whether the new credit line would replace existing credit commitments.]

[Edgy after several weeks of merger fever on Wall Street, most analysts hesitated to speculate what Texaco would do with the funds. But there was unanimous agreement that Texaco had no need for a $5.5 billion credit for its ongoing operations.]

[Analysts refused to rule out that Texaco itself may be a takeover target. "What we see here is acquisition fever," said Fliakos, the Merrill Lynch oil analyst. "Companies are looking for undervalued stock. As long as Texaco's undervalued, it's an acquisition candidate."]