New York, May 4 -- The tankers have been arriving from all over the world in unprecedented numbers. After loading up in Alaska and along the US Gulf Coast, they head back out to sea - to Japan, Thailand and Australia. All told, over the past nine weeks, more than 250 million barrels of crude from oil wells and storage tanks across the US have been shipped overseas. That's made the country, once again, the No. 1 exporter of crude, overtaking Saudi Arabia, and turned it into a lifeline for global consumers as the near closure of the Strait of Hormuz throttles West Asian supplies. But record US exports also come with warnings that this supply cushion is rapidly being pushed to its limits. Experts are questioning how long shipments can be susta...