Various classifications and specifications for hydraulic fluids are prescribed, that were developed by standardization organizations, whose demands define quality levels and appointed methods for its testing. Hydraulic fluids quality demands are defined by constructors and manufacturers of hydraulic equipment (OEM - Original Equipment Manufacturer).Development of specifications for hydraulic oils, in this case aimed for heavy machinery, are in accordance with changes in equipment construction for hydraulic systems, especially pumps, hoses and filters materials and changes in reservoir volume for hydraulic fluids. In this paper we presented the rise in demands trends for heavy machinery hydraulic oils quality, shown through changes in fluid composition and its functional characteristics.