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Brent-Dubai Exchange of Futures for Swaps (EFS) for June was valued at $3.65 a barrel, the sources said, up 28 cents from Thursday's close.
This is the widest since Jan. 14, according to Reuters data.
A wider spread makes Dubai-linked grades more attractive than those priced on Brent.
It also discourages the flow of Brent-linked crude from the Atlantic Basin to Asia.