The flow rate for the HSLIS M65 is 100 mL/min and the maximum pressure is 100psi. The actual pressure at the inlet will vary with fluid viscosity.
There is no "optimum" pressure; it just needs to be at the right flow rate.
For more information on liquid particle counting read Statistical Analysis of Particle Instruments for Liquid-borne Particles: Understanding the Impact of Size Sensitivity and Sample Volume.