Taiwan's state-run Chinese Petroleum Corpsaid yesterday it would gradually phase out its premium gasoline and replace it with 98-octane unleaded gasoline in 2000 to help reduce pollution. All production and selling of the leaded premium gasoline would be stopped beginning 2000, a spokesman for the state oil corporation said by telephone. Chinese Petroleum would introduce the new 98-octane unleaded gasoline beginning June 1, the spokesman said. "Both premium and 98 unleaded gasoline will co-exist for six months before the leaded product is removedfrom the market," he said.