* Sweet/sour spread moves in to less than $3 * LLS trades between $12.20 and $13.25, down from Friday's $13.70 * Mars trades at premium of $10.50, flat with Friday * Transatlantic spread moves out 66 cents By Janet McGurty NEW YORK, May 14 (Reuters) - U.S. cash crude differentials were weaker for light sweet crudes on Monday as two cargoes of 550,000 barrels of light sweet, domestic crude offered off the Seaway pipeline were seen pushing down the differential of the Gulf's benchmark cash crude. Changing relationships between sweet and sour crudes are seen as a result of the reversal of the Seaway crude pipeline around May 17, which initially will carry 150,000 bpd of crude out of the oil hub of Cushing, Oklahoma to Gulf Coast refineries. The first sweet crude on offer from the long-awaited first of several pipelines to ease the Cushing glut has compressed the price difference between Mars sour , a light sour blend with Louisiana Light Sweet . "It was also partly the arb combined with a lack of aggressive buying," said John Troland, independent oil advisor based in Houston. "At one point, the sweet/sour differential was inside $3 for sweet," he added. Light Louisiana Sweet between sold between $12.20 and $13.25 a barrel, well below the $13.60 a barrel over West Texas Intermediate seen Friday. Mars sour held its value, trading at Friday's level of $10.50, weaker by 80 cents. The transatlantic spread widened 66 cents to $16.79 in favor of Brent, which should have supported stronger sweet crude premiums. Traders said planned reversal of Seaway pipeline starting May 17 to bring the first crude directly from Cushing, Oklahoma, to the Gulf Coast could be having an effect. Domestic sweet has been discounted sharply from Brent because it hasn't been able to reach the world market at the Gulf Coast. When that changes, it's expected to strengthen WTI and weigh on cash crudes.