Viscosity of Hydraulic Oil

The "hydraulic oil viscosity index" should be what is specified by the manufacturer of the equipment.

I agree the manufacturer's recommendations should be followed.

Relating the different viscosity standards can be difficult. Lubricants that meet ISO 460, AGMA 7EP, and SUS 2200 may have similar properties at similar temperatures. To understand the effect of temperature, it would be best to look at the specific manufacturers specifications for the lubricant you are considering. The viscosity changes greatly with temperature.

The Globalspec search engine can take you to manufacturers sites that have their lubricants viscosity ratings at different temperatures. There are some very nice charts that roughly relate ISO to AGMA to others at various temperatures. The equipment manufacturer probably gives a recommendation for at least one of the standards.

You ask what the "viscosity index" should be. Is this correct or are you looking for what the viscosity should be at some temperature?

Viscosity index is a number that indicates the temperature dependency of the oil's viscosity ie the ability of the oil to maintain its viscosity over a range of temperatures, the higher the index the greater this ability.

If your equipment is always at the same temperature then a low index oil of the right viscosity is fine, if you have large changes in ambient or working temperature then a higher index is desirable.

Common good quality industrial hydraulic oils have a viscosity index around 100 and viscosities ranging from 22 to 100. Special "multi-grade hydraulic oils will have a viscosity index improver in the additive package to take the viscosity index to around 150.

For the correct viscosity, talk to the OEM.