This could be Well-Wisher, a short story by Bob Shaw that was indeed published in The Magazine of Fantasy & Science Fiction in November 1979.

The protagonist is Ibn Zuhain, the "Lord of the Long Valley", who finds a genie in a bottle by his fountain. He asks for three wishes, the first to heal his wounded hand, and the second is to see his land a thousand years in the future. As described in this summary:

He’s shocked to see his kingdom is no more, and instead it’s all desolate ruins and starving people being treated by the red cross. He notices cars and asks what makes them go, and the djinn explains there are magic blue crystals of great power. These are not found in the king’s land, but make the future great and allow even the poor to live as princes.

The king demands the djinn remove all crystals from this world and instead give him an equal talisman of power. He exits his garden thinking what the djinn will have given him, and finds his fountain now runs with crude oil.

or as Ibn Zuhain phrases it "The clear water of the fountain — solace of his fading years — had dried up, and in its place there gouted forth a black and evil-smelling oil which, already, had begun to disfigure everything in its vicinity."

This could be Well-Wisher, a short story by Bob Shaw that was indeed published in The Magazine of Fantasy & Science Fiction in November 1979.

The protagonist is Ibn Zuhain, the "Lord of the Long Valley", who finds a genie in a bottle by his fountain, and so is rewarded with three wishes. The first to heal his wounded hand, and the second is to see his land a thousand years in the future. As described in this summary:

He’s shocked to see his kingdom is no more, and instead it’s all desolate ruins and starving people being treated by the red cross. He notices cars and asks what makes them go, and the djinn explains there are magic blue crystals of great power. These are not found in the king’s land, but make the future great and allow even the poor to live as princes.

The king demands the djinn remove all crystals from this world and instead give him an equal talisman of power. He exits his garden thinking what the djinn will have given him, and finds his fountain now runs with crude oil.

or as Ibn Zuhain phrases it "The clear water of the fountain — solace of his fading years — had dried up, and in its place there gouted forth a black and evil-smelling oil which, already, had begun to disfigure everything in its vicinity."

The magazine issue is available from the Internet Archive.