Insulating oil contained in equipment such as transformers, circuit-breaking devices, regulators, series and shunt reactors, as received from the manufacturer and as filled at the installation site but prior to service operation, should exhibit certain properties in order to insure satisfactory performance. I t is expected that oil contained in equipment as received from the manufacturer when properly sampled from such equipment usually exhibits characteristics slightly different from those obtained from new oil, which has not been in contact with apparatus constructional materials. Additionally, certain essential properties of insulating oil, used in transformers and circuit-breaking devices, must be retained in service if the oil is to perform its multiple role of electrical-insulating, arc-quenching, and heat-transferring agent reliably. It must have adequate dielectric strength to withstand the electric stresses imposed in service. I t 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 evaluating oil received in equipment and as filled at the installation site and to assist also in his efforts to maintain his oil in serviceable condition. It recommends and standardizes oil test and ^valuation procedures, methods of reconditioning and declaiming oil when necessary, and routines for restoring oxidation resistance by additions of inhibitors