Walk into any plant store in India and you will find hydraulic oil drums marked 32, 46 or 68. The number is not a quality rating. It is the oil's viscosity grade, and picking the wrong one is one of the most common causes of noisy pumps, sluggish cylinders and oil that runs too hot.
ISO VG 46 is the default for most industrial hydraulic systems in India. Choose VG 68 when the oil regularly runs above about 60 °C, for heavily loaded presses or for older, worn pumps. Choose VG 32 for cold starts, low-pressure systems or where the machine maker specifies it. The machine manual always overrides any rule of thumb.
ISO VG stands for ISO Viscosity Grade (ISO 3448). The number is the oil's mid-point kinematic viscosity in centistokes (cSt) at 40 °C, and every grade allows ±10%. So a VG 46 oil measures between 41.4 and 50.6 cSt at 40 °C.
Viscosity falls sharply as oil heats up. A VG 46 oil that is 46 cSt at 40 °C may be close to 10 cSt at 80 °C. What matters for the pump is the viscosity at the temperature the oil actually runs at, not the label.
Hydraulic pumps are designed to work within a viscosity window. Most pump makers want roughly 16 to 36 cSt during normal running, never much below 10 cSt when hot, and not too thick at start-up.
| Situation | Usual grade | Why |
|---|---|---|
| General plant hydraulics, oil 40–60 °C | VG 46 | Fits most vane, gear and piston pumps |
| Presses, high loads, oil often above 60 °C | VG 68 | Holds film strength when hot |
| Injection moulding machines in hot sheds | VG 46 or 68 (as per maker) | Many run 50–60 °C; check the manual |
| Older pumps with internal wear | VG 68 | Thicker oil reduces internal leakage |
| Cold starts, low-pressure or precision systems | VG 32 | Flows easily and responds faster |
| Mobile equipment working outdoors | VG 46/68 or a high-VI oil | Wide temperature swing between morning and afternoon |
Many machines in Indian sheds see 40–45 °C ambient temperature in May and June. Oil tanks that run at 50 °C in winter can reach 65–70 °C in peak summer, especially where coolers are undersized or radiators are blocked by dust.
If your oil temperature goes up by 15–20 °C between seasons, a VG 46 oil may thin out beyond the pump's comfort zone. In that case you have three options:
Topping up a VG 46 system with VG 68 gives an unknown viscosity and may mix additive systems. If you change grade, drain and flush, and label the tank clearly.
Two VG 46 oils can perform very differently. Anti-wear additives, oxidation stability, air release, water separation and filterability all matter. That is why the oil type (HL, HLP, HVLP) and the specification behind it are just as important as the number. We explain this in Are all ISO VG 46 hydraulic oils the same?
Keep the oil clean as well. Most hydraulic failures start with dirt and water, not viscosity. The ISO 4406 cleanliness code tells you how clean is clean enough.
Can I use VG 68 instead of VG 46 in my hydraulic system?Often yes for hot-running or heavily loaded systems, but check the pump maker's minimum and maximum viscosity first. In cold starts a VG 68 oil can starve the pump.
What is the difference between hydraulic oil 46 and 68?Viscosity. VG 68 is about 50% thicker at 40 °C (61.2–74.8 cSt) than VG 46 (41.4–50.6 cSt). The additive type can be the same.
Is hydraulic oil 46 the same as HLP 46?"46" is the viscosity grade. "HLP" is the oil type under DIN 51524 Part 2, meaning it has anti-wear and anti-oxidation additives. HLP 46 is a VG 46 oil of that type.
How hot should hydraulic oil run?Most systems are happiest between 40 and 60 °C. Above about 60 °C oil ages faster; as a rule of thumb, oxidation roughly doubles for every 10 °C rise above that.
Not sure whether your press or moulding machine should run on 46 or 68? Send us the machine make, pump type and summer oil temperature and our team will recommend the grade.