LOOPHOLE FOR TANKERS IN NORWAY'S S. AFRICA BAN Norway's parliament has approved an extensive trade ban against South Africa but left shipowners a key loophole through which controversial oil shipments on Norwegian tankers may continue, government officials said. The unilateral boycott gives domestic companies until late September to cut trade ties with South Africa and Namibia. Although forbidding crude oil shipments to South Africa on Norwegian-owned tankers, the boycott makes a crucial exception for ships whose final destination is decided while at sea. As oil cargoes are often resold after loading, critics said the door will be left open for continued shipments to South Africa. Norwegian tankers supplied South Africa with about 30 pct of its crude oil imports during the early 1980s, but the trade has dropped sharply to just one cargo in the last three months, trade ministry officials said. The latest trade figures show Norwegian imports from South Africa dropped 36 pct to 160 mln crowns during the first eight months of 1986, while exports plunged 52 pct to 265 mln crowns from the year-ago figure. The boycott legislation now goes to the upper house for formal ratification later this week, parliamentarians said.