SINGAPORE : Oil prices turned lower in Asia on Monday, weighed by a slowdown in China's manufacturing sector and spending cuts in the United States, analysts said.New York's main contract, light sweet crude for delivery in April dropped 29 cents to $90.39 a barrel in the afternoon and Brent North Sea crude for April delivery shed 17 cents to $110.23. China's official purchasing managers' index released Friday showed growth in manufacturing activity slowed last month, suggesting a recent pick-up in the world's number two economy is weaker than initially thought.