Does a slight film of oil on the surface of the water render the wa- ter impure?

Answer

The slight film of oil on the surface of the water will not eliminate the Tuhurriyah of the water according to the Maaliki Math-hab. The water remains fit for wudhu. Only if the oil becomes mixed with the water, i.e. the oil is diffused into the water, will wudhu not be permissible with such water despite the water remaining Taahir (pure).

Tuhuriiyah is the cleansing property of the water. It is water that can purify impurified items. Taahir means clean. While it remains clean, it has lost its property of cleansing impurified items.