By Lukeron Lubricants · Last updated
Hydraulic oil comes in three grades that buyers ask for most: VG 32, VG 46 and VG 68. They look the same in the drum, but they flow very differently. The wrong grade does not fail on day one. It shows up later as heat, noise, slow machines and a worn pump. This guide explains what the number means and how to pick the right grade for your machine.
What does VG 32, 46 or 68 mean?
VG stands for Viscosity Grade, from the ISO VG system. The number is how thick the oil is at 40 °C, measured in centistokes (cSt). VG 32 is about 32 cSt, VG 46 is about 46 cSt and VG 68 is about 68 cSt. A higher number means a thicker oil. The number does not tell you the quality of the oil or the additives in it. Those come from the oil type, such as AW (anti-wear) or HVI (high viscosity index).
The three grades at a glance
VG 32: the thinner oil
VG 32 flows easily, so it suits cold starts and cooler places. It is common in outdoor equipment in cold weather, in systems with a fine fit between parts, and where the machine maker asks for it. In a hot machine it can become too thin.
VG 46: the usual choice
VG 46 is the grade most machines use when the oil runs at a normal temperature. If the machine maker has not said otherwise and the shop floor is not very hot or cold, VG 46 is the common starting point.
VG 68: the thicker oil
VG 68 keeps a stronger oil film when the oil is hot or the pressure is high. It is used in hot workshops, heavy presses and machines that run for long hours. In cold conditions it can be slow to flow at start-up.
How to choose: three steps
- Start with the machine maker. The manual or the plate on the machine states the viscosity grade for the pump. This is the first answer, and it overrides every rule of thumb.
- Look at the oil temperature, not only the weather. Check how hot the oil runs when the machine is working. Hot oil gets thinner, so a machine that runs hot may need a thicker grade. A machine that starts in the cold may need a thinner one.
- Check the pump and the pressure. Different pump types (vane, gear, piston) have a viscosity range they work best in. High-pressure systems usually need a thicker oil to hold the oil film.
What goes wrong with the wrong grade
Oil too thin
The oil film between metal parts becomes weak. You may see more pump wear, internal leakage inside the pump, slower machine movement and a rise in oil temperature.
Oil too thick
The pump has to work harder to push the oil. You may see slow response at start-up, a noisy pump, more energy use and heat. In very cold start-ups the pump may not get enough oil at once.
In both cases the damage builds up slowly, so it is easy to blame something else.
Do not mix grades or oil types casually
Topping up VG 46 with VG 68 gives you something in between, and the oil may no longer match what the machine maker asked for. Mixing different oil types, such as an AW oil with another type, can also upset the additives. When you change grade or type, ask your supplier first, and drain and flush as the machine maker advises.
What is HVI hydraulic oil?
HVI means high viscosity index. The viscosity index tells you how much the oil thins out when it gets hot and thickens when it gets cold. A higher index means the oil stays closer to the same thickness across a wider temperature range. HVI oils are used where the machine works in changing temperatures, for example outdoor and mobile equipment.
Quick reference
- VG 32: cooler conditions, cold starts, where the maker asks for a thin oil.
- VG 46: normal operating temperature, general industrial hydraulics.
- VG 68: hot conditions, high pressure, long running hours.
- If the manual gives a grade, use that one.
For a closer look at the two most used grades, read our guide on hydraulic oil AW 46 vs AW 68.
Lukeron hydraulic oils
Our anti-wear hydraulic oil range is available in ISO VG 32, 46 and 68, with HVI options, in 20 litre and 210 litre packs. The ECO hydraulic oil, for small factories, workshops and agricultural machinery, comes in VG 32, 46 and 68 in 5 litre to 210 litre packs. Tell us your machine, pump type and working temperature, and we will help you choose, and send a free sample to test. Follow the machine maker's grade and cleanliness requirements.