Look at this, it may be helpful for your topic. Good luck.

http://www.machinerylubrication.com/Read/25967/hydraulic-oil-lubrication-viscosity

Dear colleague Azam,

Suitability of hydraulic fluid for specific application depends on the following factors: Viscosity and Viscosity-Temparature behavior, Wear protection (FZG gear test,Vane pump test), Compatibility with construction materials (coatings, seals, copper alloys, plastic), resistence to: thermal, chemical and mechanical stresses, air separation ability, demulsifying ability and water solubility, corrosion protection, filterability, foaming behavior, environmental requirements.

Colleague Didier Vuarnoz explained the division of hydraulic fluid to the ISO specification, a specification DIN is often applied, also.

 DIN specifications:

- DIN 51524-1 HL. Hydraulic fluids based on mineral oils and related hydrocarbons (HL- fluids can be used in hydraulic systems that do not pose any requirements as to wear protection, HLP- fluids (DIN 51524-2) are suitable for most fields of application and components provided the temperature and viscosity provisions are observed, HLPD (DIN 51524-2) and HVLPD- fluids (DIN 51524-3) are used in systems where deposits as well as solid or liquid contamination need to be kept temporarily suspended, HLPD and HVLPD fluids are used in systems where deposits as well as solid or liquid contamination need to be kept temporarily suspended, HVLP fluids (DIN 51524-3) are used in systems operated over a wide temperature range (Index Viscosity >140).

Dear colleague Azam

If you are making and oil selections for best performance on Hydraulic Oil  (A,B, C etc…) I recommend you to divide the research process in 3 set of assessment.

1- Lab test (See all the previous recommendations above)

2- Bench Test

3-Field test. (Tis is the most important)

 For bench test you can use a  4 ball machine ASTM D4172 , ASTM D2266 and D4172 comparing and measuring wear, friction coefficient, welding point etc. to quantifies the wear protection at increasing loads. Also you can compare among your samples which lubricant performed better during the transition from elastohydrodynamic to boundary lubrication and metal to metal contact.

Other important bench test for Hydraulics lubricants is the comparison for Shear Stability. ASTM D5621. Hydraulics lubricants in real applications have to pass through small diameters  where the molecules are sheared degrading the viscosity.

The most determinant test for a good selection will be your battery of field test. Because lubricants are produce under high level of standards and they have manufactured with very similar physic and chemical properties, most of them have similar results on lab tests. However   in the field will be a lot different variable as ventilations, volatile contaminant, water ingression in the system, filters conditions, reservoir and storage quality, wear debris, machine conditions, ambient temperature etc. Each one of your Hydraulic sample would have different performance on the field conditions.