- Soluble Oil is a water-soluble metal fluid that forms a very stable emulsion when mixed when mixed with water at a wide range of dilutions; is fortified with biocides to prevent rancidity caused by bacteria, which can lead to unpleasant odors, split emulsions and reduced product performance. -Soluble Oil is recommended for light to moderate duty machining operations on all ferrous and non-ferrous metals except magnesium.