The PM4S Vane Motors are recommended when system back pressure is over 2000 PSI and the motor exceeds 2000 RPM. Also if fluid viscosity is less than 115 SUS(24 cSt) and the motor speed is exceeds 2000 RPM, the PM4S version is recommended. This special version of our PM4C motor will produce higher efficiency and enjoy greater product life under these demanding conditions.