Most hydraulic failures come from the fluid: wrong viscosity, wrong additive or contamination. Finding the right ISO VG, knowing the HL/HM/HV difference and setting a cleanliness target.
In a hydraulic system the fluid is not just a liquid; it is a working component that transmits force, lubricates parts, carries heat and protects against corrosion. Most pump and valve failures come from the wrong fluid or a dirty fluid. The right choice comes down to three questions: which viscosity, which additive class, which cleanliness level?
1. Viscosity: the ISO VG grade
Hydraulic oils are named by grades such as ISO VG 22, 32, 46, 68, 100; the number is the viscosity at 40 °C. The right grade is chosen from the pump maker's allowed viscosity window and the operating temperature. Rule of thumb: ISO VG 46 for indoor equipment at steady temperature, ISO VG 32 or a multigrade (HV) oil for outdoor mobile equipment that sees seasonal swings. Too thin increases internal leakage and wear; too thick increases cavitation and energy loss.
2. Additive class: HL, HM, HV, HFC
- HL: base oil with rust and oxidation inhibitors; lightly loaded, low-pressure systems.
- HM: HL plus an anti-wear additive; the standard for industrial and mobile systems and for high-pressure pumps.
- HV: HM with a high viscosity index; viscosity changes little over a wide temperature range, for outdoor equipment.
- HFC / HFDU: fire-resistant fluids for high fire-risk areas (foundries, steel plants); they need compatible seals and paint.
3. Cleanliness: the ISO 4406 target
The life of a hydraulic system depends directly on the amount of solid particles in the fluid. The cleanliness level is expressed by an ISO 4406 code (e.g. 18/16/13); a lower code means cleaner oil. New oil straight from the drum is usually dirtier than the target and should be filtered as it is added. Systems with servo or proportional valves want 17/15/12 or cleaner; 19/17/14 is a typical target for a standard industrial system. Filter maintenance and moisture control are how that level is held.
Changing and mixing oil
Mixing oils of different brands or different additive chemistry can make additives drop out and form sludge. When changing brand, the system should be drained and flushed. The change interval should be set by oil analysis, not by the calendar: change in viscosity, acid number (TAN), water content and particle count tell you the fluid's remaining life. In a well-maintained industrial system the oil can serve for years.
In short
The right hydraulic oil is one in the ISO VG grade the pump allows, in the additive group that suits the system pressure (HM for most systems, HV outdoors) and at the cleanliness level the equipment requires. When all three are met, pump and valve life is noticeably longer. C-Oil hydraulic oils are produced in a range of ISO VG and additive classes; we can work out the right product for your system's data together.