Viscosity must be maintained within limits al the operating temperature. Petroleum based Hydraulic oil with a high viscosity index (95 or higher) and a viscosity grade of ISO VG 46 or 68 is optimum.

	Optimum Viscosity, SUS at 100 degrees F	Fluid Cleanliness Levels ISO/DIS 4406 (SAE J1165)
Gear Pump	200-300	19/17/14
Piston Pump	200-300	18/16/13
Geroller Motor	200-300	19/17/14
Vane Motor	200-300	19/17/14
Mobile Spool Valve		19/17/14
Cartridge Valve		18/16/13

The fluid cleanliness levels described can be achieved in systems with normal dirt ingression by filtering one third of the total fluid volume in the system per minute across filter media of suitable dirt holding capacity and with a pore rating of 10 micron absolute (Beta 10 equal or greater than 200).