The spill has emerged off Gotska Sandon, where the 3,000-gt Immen went down 37 years ago en route to Lubeck.
The wreck has been located 133 metres down by a remote-operated vehicle (ROV).
Heavier oil will be scooped up by environmental protection ships, while lighter fuel will evaporate.
There is no risk of the slick reaching the coast, the coast guard said.
