GB/T 12578-1990 describes the U-shaped tube method for the determination of lubricating oil flowability. This standard clarifies the principle of measurement and judges the flowability of lubricating oil by using the flow state of lubricating oil in the U-shaped tube at different temperatures. Specifications and requirements of U-shaped tubes and other instruments required for testing are clarified. The preparation process of lubricating oil samples is specified in detail, including sample collection, processing, etc., and the operation steps such as temperature control, observation records, and result determination are also described. It provides a unified method for the accurate determination of lubricating oil flowability, which helps to evaluate the suitability of lubricating oil under different working conditions.