Certain essential properties of insulating oil, used in transformers and circuit breaking devices, must be retained in service if the oil is to reliably perform its multiple role of electrical insulating, arc quenching and heat-transfer agent. It must have adequate dielectric strength to with stand the electrical stresses imposed in service. It must retain a sufficiently low viscosity so that its ability to circulate and transfer heat is not impaired. It must pour readily at low temperatures, and have high flash point and fire point for safety. Its dielectric losses should not become excessive. It should not be allowed to become so deteriorated or contaminated that it adversely affects other materials in the apparatus, nor should deterioration products sludge it sufficiently to impair its circulation through cooling ducts. This guide attempts to assist the power equipment operator in his efforts to maintain his oil in serviceable condition. It recommends and standardizes oil test and evaluation procedures, methods of reconditioning and reclaiming oil when necessary, and routines for restoring oxidation resistance by additions of inhibitors.