Last July, E. I. du Pont de Nemours & Co. was a well-regarded producer of fibers, chemicals and high-technology products, selling at $51 a share plus change on the New York Stock Exchange.

Seagram Co. was a giant Canadian-based distiller with a thick wallet and a hungry eye.

And Conoco Inc., the ninth-largest American petroleum company, whose stock was selling at an undervalued price of $69 a share on July 2, was an irresistible full-course meal.

These three produced the largest corporate merger ever, when Du Pont bought Conoco for $7.8 billion on Sept. 30, and Seagram, a loser in the bidding, wound up with a 20 percent stake in Du Pont.

Six months later, with many of Du Pont's core businesses hard hit by recession and Conoco's revenues undercut by a worldwide oil glut, the wisdom of the merger still is being debated by Wall Street analysts who follow the two companies.

"Most of them really don't like this merger very much," according to Thom R. Brown of Butcher & Singer in Philadelphia. He is one of the exceptions, saying he thinks it makes good sense as a long-term strategy for the 1980s and '90s.

The only immediate winners, it seems, were the Conoco shareholders who accepted Du Pont's offer of cash for their stock. (Du Pont was willing to buy 40 percent of Conoco's shares for cash; it swapped Du Pont stock for the remainder.)

Those who took the cash offer got $98 a share, twice what Conoco stock was selling for in May, before the bidding began. (The windfall was subject to income tax, however.)

Seagram lost the bidding but swapped its Conoco shares for Du Pont stock, obtaining 47 million shares, a bigger bloc than the Du Pont family holds. The advantages of becoming a major owner of Du Pont have not yet been demonstrated for Seagram's shareholders.

The decline in Du Pont's stock price since last summer from $51 a share to $32 last week has reduced the value of Seagram's stake. It paid $2.6 billion for its piece of Du Pont, an investment that is now worth $1 billion less. And the Conoco shareholders who received Du Pont stock have had little to cheer about.

In Du Pont's eyes, however, this kind of instant analysis is pointless. "You never make these moves in the short term," says Edward G. Jefferson, Du Pont's chairman and the matchmaker of the Conoco merger. "If that were the reason, it wouldn't have been a good one."

A more fundamental debate is under way, however, about the long-term value of the Du Pont-Conoco merger. That debate begins with disagreements over the future course of oil prices, so central to Conoco's performance.

John Henry of E. F. Hutton calls the merger a "disaster" for Du Pont and Seagram. Henry says Du Pont is looking for oil prices to rise at a rate 2 or 3 percent faster than inflation, whereas he expects a continuing decline for several years. If oil prices do drop further, the damage to Conoco's earnings would be a drag on the earnings of Du Pont's promising businesses, Henry says.

Moreover, he expects depressed oil prices to cool off demand for coal, hampering Jefferson's plan to reduce the debt Du Pont took on to make the Conoco acquisition. Du Pont borrowed nearly $4 billion, doubling its traditionally conservative debt-to-equity ratio to 40 percent, and Jefferson's goal is to cut that in half by 1985.

He will have trouble, predicts Henry. "I don't think it will work out too well. It looks like they bought at the top of the market."

Brown, on the other hand, argues that the recent agreement among members of the Organization of Petroleum Exporting Countries to limit production shows that the oil cartel hasn't folded its tents and that the possibility of future leaps in oil prices remains very real.

The tremendous quantities of petroleum and natural gas that Du Pont requires for its chemicals and fiber businesses make the merger desirable in the long run, says Brown.

A second issue is whether the value of Du Pont stock was diluted by the merger, which required Du Pont to issue 78.5 million additional shares of its stock, worth $3.9 billion at the time, in exchange for the Conoco shares. That brought the total number of Du Pont shares outstanding to 184.2 million.

The results since the merger show that there was no dilution, according to Jefferson.

The addition of Conoco improved Du Pont's earnings per share despite the additional interest expense due to the merger debt, Jefferson says. "And that condition continues."

Henry and some other analysts doubt that Conoco will be a plus for Du Pont during the next six months or more.

But the deal wasn't meant to be justified on the basis of what happened over the first six months, Jefferson argues.

Even with a conservative view of future energy prices, Du Pont got Conoco's oil, gas and coal reserves for 55 cents on the dollar, Jefferson says. If oil prices rise significantly, so will Conoco's profits. If petroleum supplies run short again, Du Pont has a captive supply. If petroleum prices remain level or decline, Du Pont should benefit from cheaper raw-material costs, Jefferson says.

Brown agrees. But he says that nine out of 10 market analysts believe that the addition of Conoco will slow Du Pont's overall growth. The breakthroughs Du Pont is pursuing in fiber research and genetic engineering, for instance, will have a much less dramatic impact on the sales of the combined companies than they would have had on the pre-merger Du Pont.

That negative perception could be altered by a successful sale of some of Conoco's assets this year, Brown says. Beyond that, Brown and other analysts will watch closely to see how Du Pont chooses to spread its capital investments among Du Pont's old businesses and Conoco's activities.

That is a fundamental strategic question that Du Pont's top officers are addressing, says Jefferson. "I'm not sure how it will look," he says. But come what may, Du Pont will not pull back on its commitment to developments in high-technology areas, Jefferson insists.

"You have to nurture the new discoveries. Otherwise, why spend $750 million on research?" he asks.