Oil prices slipped in quiet trading as traders showed reluctance to take positions either way in a still volatile market. On the New York Mercantile Exchange, West Texas intermediate--the benchmark U.S. crude for immediate delivery--dipped 4 cents to $20.16 a barrel. Britain's North Sea Brent crude lost 20 cents to $18.90 a barrel. On the U.S. Gulf Coast spot market, West Texas intermediate fell 20 cents to $19.90 a barrel. Petroleum markets also appeared to be taking note of world oil inventories, which have been building up in the last several months.