A TWO-PHASE programme will be introduced to reduce the sulphur content of diesel oil by 10 times its present level by 1998, principal Environmental Protection Officer for Air Services, Raymond Leung Pak-ming, said yesterday. According to the Government's schedule, the sulphur level will be cut to 0.2 per cent in 1995 and further reduced to 0.05 by the year 1998. Diesel oil is commonly used by lorries and buses in Hong Kong. Mr Leung said the Government had decided to cut the sulphur amount gradually, after taking into account developments in diesel engine technology. But legislators and environmentalists cast doubts on the effectiveness of the move in reducing pollution. Speaking at the Legislative Council's environmental affairs panel, the Reverend Fung Chi-wood of the United Democrats described the Government's approach as ''too conservative''. ''I think a less conservative approach should be adopted. We are not saying that the Government had to cut the level of sulphur overnight, but why should it take so long?'' Independent legislator Peggy Lam Pei Yu-dja also questioned the time schedule set by the Government. ''Hong Kong has to wait until 1998 before it gets the sulphur level [of diesel oil] lowered to that of the United States. That is five years to go. I really cannot understand why we have to wait another five years while the US is adopting it now.'' But Mr Leung said the American standard was so rigid that even advanced countries like Japan could not meet it. The Conservancy Association welcomed the move, but cast doubts on the implementation of the new regulations. Association spokesman Hung Wing-tat said it would be difficult for the Government to introduce checks on the oil quality, as more and more truck drivers have their tanks filled on the mainland.