S-Oil Corp., South Korea's third-largest refiner, said Thursday its net profit tumbled 82.6 percent in the first quarter from a year ago due to lower cracking margin and a drop in paraxylene (PX) spread. Net income came to 25.59 billion won (US$24.7 million) in the January-March period from 147.45 billion won a year earlier, S-Oil said in a regulatory filing.