A silicone oil is any liquid polymerized siloxane with organic side chains. The polymer backbone consists of alternating silicon-oxygen atoms (...Si-O-Si-O-Si...).

Silicone oils are rated on their viscosity with the shorter chains giving the lower viscosity. Viscosity is measured in Centisokes (CS) with 1 CS = the viscosity of water.

S5 has a viscosity 5 times that of water

Silicone oils are primarily used as lubricants, thermic fluid oils or hydraulic fluids. They are excellent electrical insulators and, unlike their carbon analogues, are non-flammable. Their temperature-stability and good heat-transfer characteristics make them widely used in laboratories for heating baths ("oil baths") placed on top of hotplate stirrers, as well as in freeze-dryers as refrigerants. Silicone oil is also commonly used as the working fluid in dashpots, wet type transformers, diffusion pumps and in oil-filled heaters.

Silicone oils are also used in cosmetic creams to reduce the whitening effect while rubbing in.