Can ISO VG 46 and VG 68 Hydraulic Oils Be Mixed?

What Happens If ISO VG 46 and VG 68 Hydraulic Oils Are Accidentally Mixed?

In one factory, an operator carelessly poured half a barrel of ISO VG 46 oil into a hydraulic tank filled with VG 68 oil. 3 days later working, the hydraulic equipment completely broke down-oil temperature surged to 80°C, and pressure gauge needles fluctuated erratically. When the maintenance opened the pump, they found that the plunger pump was clogged with gelatinous deposits, and they had to clean the pump and replace the entire sealing system. So why can't different viscosity grades of hydraulic oil be mixed?

What do 46 and 68 mean?

46 & 68 are actually its kinematic viscosity at 40°C (unit: mm²/s).
• 46 oil: 40°C viscosity = 41.4-50.6mm²/s, equivalent to the fluidity of honey at 30°C
• 68 oil: 40°C viscosity = 61.2-74.8mm²/s, close to the state of just melted butter

What will happen if these two hydraulic oils are mixed?

Mixing the 2 VG viscosity hydraulic oils will cause the viscosity-temperature characteristics of the oil to be unbalanced.

Mixing hydraulic oils of different viscosities (such as VG 46 and VG 68) may cause a serious imbalance in the viscosity-temperature characteristics of the oil. The viscosity index (VI) deviation makes the oil thicker at low temperatures and thinner at high temperatures, destroying the strength of the lubricating film. The system may have problems such as cold start difficulties, high-temperature leakage, and unstable pressure, which accelerate the wear of pumps and valves. The oil must be completely changed, and the system must be cleaned to restore stable performance.

Additive incompatibility

Hydraulic oil additives of different brands (such as anti-wear additive ZDDP and antioxidant amines) may have chemical compatibility issues. If mixed, the active ingredients may react, resulting in flocculent precipitation or even colloids in the oil. These impurities will clog the filters (in actual cases, the life of 5μm filter elements can be shortened by more than 60%) and adhere to the surface of the valve core to cause jamming. Typical manifestations are system pressure fluctuations and increased pump noise. It is recommended that DO NOT mix hydraulic oils of different brands.

Proper oil selection ensures system reliability and longevity. The users should strictly adhere to the manufacturer's specified oil grade. Mixing oils may cause severe hydraulic equipment damage, leading to costly maintenance/repairs far exceeding the price of hydraulic oil.