Cranes use hydraulic oil to exploit the principle of ‘moving fluids’ and ‘fluid leveraging’.
What it means is that, instead of, for example, using pure mechanical lever or screw in utilising mechanical advantage of turning big movement with less loading into a small movement with smaller loading, it rather pushes , via hydraulic pump, bigger amount of pressurised hydraulic oil under smaller load to make smaller movement of same pressurised oil under bigger load.
Why oil?
Simple, oil doesn’t cause rust, as the fluid is in metal enclosure at most of the time (except when in high-pressure hoses).
To be able to use oil, it also needs to retain some sort of stable temperature and purity.
Oil circulates through coolers, similar to car radiator/cooler.
As oil circulates in and around moving metal parts, it is exposed to absorbing any metal shavings and other impurities. For that reason, oil also goes through a series of hydraulic oil filters, and is eventually replaced when it reaches it’s use-by date/hours. It is also changed earlier if there is any contamination discovered in hydraulic oil.
How hydraulic oil reaches it’s operational pressure?
It is done via Hydraulic Pump, which is again driven by crane’s own engine (usually diesel engine).
