Which type of cutting fluid is NOT intended to be mixed with water?

Straight oil is a type of cutting fluid characterized by its composition, which consists solely of oil without additives that allow it to be mixed with water. Unlike other cutting fluids, such as emulsions or water-soluble oils, straight oil provides lubrication and cooling without the need to dilute it. This makes it ideal for specific applications where a high level of lubrication is required, such as heavy machining or operations that produce a significant amount of heat.

In contrast, synthetic fluids, emulsion oils, and water-soluble oils are formulated to be mixed with water to enhance their cooling and lubricating properties. Emulsion oils, for instance, create a stable mixture when combined with water while retaining effective lubrication. Water-soluble oils also blend with water, improving fluidity and temperature control during machining processes. Synthetic fluids may contain various chemical compounds designed to provide optimal performance when mixed with water.

The distinction in function and composition underscores why straight oil stands out as not intended for mixing with water, making it the correct answer in this context.