A tanker carrying 8 million gallons of diesel oil broke in half during a storm yesterday and began leaking oil off the northwest coast of France. Officials at the Brest Port Authority, 60 miles away, estimated that between 600,000 and 1.5 million gallons of the oil had been spilled so far, and said it would be at least two days before there would be any risk of oil washing up on the coast. If conditions improve today, crews will try to siphon the remaining oil off the tanker.