Mineral fluids dot 3, 4, and 5.1 are all considered to be "improving" grades.
Because Dot 5 is based on silicone, its improper application will result in the destruction of rubber seals found throughout a hydraulic system.
My acquaintance in South Cheshire did an excellent job reupholstering my Firenza seat with old material from a rear seat because the cloth is unavailable.