Short answer
ISO VG 32, 46 and 68 are three adjacent viscosity grades: about 32, 46 and 68 cSt at 40 °C. ISO VG 32 is thinner and flows more easily in the cold, ISO VG 68 is thicker and holds viscosity better when the system runs hot, and ISO VG 46 sits between them. The correct grade is the one the pump or machine manufacturer specifies for your temperature range.
The numbers side by side
Under ISO 3448, ISO VG 32 allows 28.8 to 35.2 cSt at 40 °C, ISO VG 46 allows 41.4 to 50.6 cSt, and ISO VG 68 allows 61.2 to 74.8 cSt. Each grade is roughly 45% to 50% thicker than the one below it at that reference temperature.
The gap matters most at temperature extremes. A thin oil that is ideal at a cold startup may become too thin once a heavily loaded system reaches its working temperature, and a thick oil that suits summer heat may be sluggish on a frosty morning.
What the pump maker is balancing
Hydraulic pump manufacturers usually publish an allowable viscosity window: a maximum for cold starting, so the inlet does not cavitate, a minimum at operating temperature, so internal clearances keep a lubricating film, and an optimum range in between. They then recommend the ISO grade that keeps the oil inside that window for the expected ambient and tank temperatures.
Vane, gear and piston pumps have different limits, and so do motors, valves and seals in the same circuit. That is why a manual may call for ISO VG 32 in one climate and ISO VG 46 or 68 in another, or list a specific oil type such as HM or HV alongside the grade.
Climate and duty cycle
Outdoor mobile equipment such as excavators, log splitters and loaders sees the widest temperature swings, and is where multi-viscosity (HV or HVLP) oils are often specified to cover a broader range with one fill. Indoor plant systems such as presses and injection molding machines run in steadier conditions, so a single-grade anti-wear oil is common.
If the reservoir regularly runs much hotter than the design temperature, changing viscosity is not the fix; check cooling, relief settings and contamination first, because heat ages any oil quickly.
Changing grades
Moving between grades is sometimes recommended seasonally, but mixing large amounts of two grades leaves you with an in-between viscosity and blended additive chemistry. When you change grade or oil type, follow the equipment maker’s flushing guidance and label the reservoir.
Frequently asked questions
Is ISO VG 46 the most common hydraulic oil?
ISO VG 32, 46 and 68 are all widely used. Which one appears most in your fleet depends on your equipment makers and climate, not on any single industry default.
Can I use ISO VG 68 instead of 46 in hot weather?
Only if the equipment manufacturer allows it for that temperature range. A thicker oil can raise cold-start risk and change how valves and pumps respond.
What is the difference between AW 46 and HV 46?
Both are ISO VG 46 at 40 °C. An HV (multi-viscosity) oil has a higher viscosity index, so it thins less when hot and thickens less when cold than a typical HM or AW oil.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
