BP PLC warned that it could be Sunday or later before the outcome of its bid to plug the well through an effort known as a "top kill" would succeed.
Experts said they could see incremental progress at best from BP's "spillcam" of mud, gas and oil billowing from the seafloor. The hypnotic video has become an Internet sensation as Americans watch and try to fathom whether BP's efforts are working.
Scientists say the images may offer clues to whether BP is getting the upper hand in its struggle to contain the oil, said Tony Wood, director of the National Spill Control School at Texas A&M University in Corpus Christi. If the stuff coming out of the pipe is jet black, it is mostly oil and BP is losing. If it is whitish, it is mostly gas and BP is also losing.
If it is muddy brown, as it was much of Friday, that may be a sign that BP is starting to win, he said. That "may in fact mean that there's mud coming up and mud coming down as well," which is better than oil coming out, Wood said.
Philip W. Johnson, an engineering professor at the University of Alabama, said the camera appeared to show mostly drilling mud leaking from the well Friday morning, and two of the leaks appeared a little smaller than in the past, suggesting the top kill "may have had a slight but not dramatic effect."
But Bob Bea, a professor of engineering at University of California at Berkeley who has studied offshore drilling for 55 years, said late Friday that what he saw didn't look promising.
He likened the effort to pushing food into a reluctant baby's mouth - it only works if the force of the stuff going down is more than the force of what's coming up.
"It's obvious that the baby's spitting the baby food back" because the pressure pushing up from the well is stronger, Bea said.
Meanwhile, President Barack Obama visited the coast Friday to see the damage as he tried to emphasize that his administration was in control of the crisis, which began April 20 when the Deepwater Horizon drilling platform blew up and killed 11 workers.
"I'm here to tell you that you are not alone, you will not be abandoned, you will not be left behind," he told people in Grand Isle, where the beach has been closed by gobs of oil and the frustration and anger are palpable. "The media may get tired of the story, but we will not. We will be on your side and we will see this through."
He also urged the public to volunteer to join the cleanup and for tourists to help by visiting the majority of the region's coastline that is untouched.
Hundreds of workers hit the beaches ahead of Obama's visit, cleaning debris from the shoreline before they hopped on buses and left soon after the president arrived.
"This is the cleanest I've ever seen the beach," Louisiana Gov. Bobby Jindal said. "We saw a surge of activity the last couple of days. Let's hope it continues now that he's gone."
The top kill operation began Wednesday, with BP pumping heavy drilling mud into the blown-out well in an effort to choke off the source of the spill which has released anywhere from 18 million gallons to 40 million of oil by the government's estimate.
Coast Guard Adm. Thad Allen said the denser-than-water mud had pushed down the oil and gas that's forcing its way up from underground, but the mud had not overwhelmed the gusher.
BP has brought in about 2.5 million gallons of drilling mud for the top kill. BP chief operating officer Doug Suttles said Friday the procedure was going basically as planned, though the pumping has stopped several times.
