Oil used for hydraulic system to transferring pressure is generally known as hydraulic oils.
The basic function of this oil is to keep active the hydraulic or pneumatic system.
In most cases,hydraulic oil is used in hydraulic equipment in high temperature & pressure. In these case oil consumption may not high.For example, in different construction or material handling equipment such as excavator, pay loaders etc.In all these equipment have hydraulic chamber which is operates under pressure by a pneumatic system or by manual system.In these Chamber have to fill with hydraulic oil of proper viscosity grade recommended by equipment manufacturer. And this chamber is generally sealed by oil seals.If hydraulic oil selection is not right in that case oil seals may broken. In some cases pneumatic system may broken due to wrong selection of hydraulic oil.
On the other hand bulk quantity of hydraulic Oil may requires in large hydraulic system such as shearing machine. In these case high viscosity index hydraulic oil generally required.
Depends on application different specifications may requires for various hydraulic system.For common use only important specification of hydraulic oil is ISO VG.But for critical & wide range application some extra specification may needs such as high viscosity(HVI),HVLP,DIN PART 3,Cleanliness,NAS value etc.
Hydraulic Oil Checking:
As like as other maintenance hydraulic oil also need to check regularly.In brake have to check the fork either it touching the lower end or floor or not.If touches the lower end then need to add or Change the hydraulic oil.
For the tractor it is also important for checkiythe hydraulic fluid.But depending on manufacturer guidelines & engine design as different hydraulic system in different engine.If Some one ask,how to check hydraulic fluid in jhon deer tractor?
And how to check hydraulic fluid in Caterpillar tractors.The answer would not same.But basic thing of checking hydraulic system is, checking the pressure,oil seal etc.If anything found abnormal then need to change or top up the hydraulic oil.