Why would one use ASTM D 5621 or equivalent for shear testing of Hydraulic Fluids? Guidelines for the Question of the Day: 1. No Piston Cup Lapel Pin will be awarded to the same person within 14 days of a previous award. 2. Please respond with a complete sentence. 3. Please do not post links. If a question arises as to the need for clarification, then sources and links may be requested. 4. Please, no off topic posts.