Hydraulic Oil 46 vs 68: Differences and How to Choose
By the PCG Technical Desk · Updated 27 September 2026
The numbers
Under ISO 3448, VG 46 covers 41.4–50.6 cSt at 40 °C and VG 68 covers 61.2–74.8 cSt. IndianOil sets tighter limits for its grades: Servosystem 46 is 42–50 cSt and Servosystem 68 is 62–74 cSt (IOCL literature).
What really decides the grade: oil temperature
Pumps are designed to work within a viscosity window at operating temperature. A VG 68 oil running at 60 °C has roughly the viscosity a VG 46 oil has at about 50 °C. If your tank runs hot (presses, moulding machines in summer, poor coolers), VG 68 keeps a healthy film. If the system runs cool or starts cold, VG 46 flows faster to the pump inlet and wastes less energy.
| Situation | Usually |
|---|---|
| OEM manual specifies a grade | Always follow the OEM |
| CNC machine hydraulics, controlled shop | VG 46 |
| Injection moulding / presses running hot | VG 68 |
| Cold starts, outdoor mobile equipment | VG 46 or a high-VI oil |
Don't upgrade viscosity to fix a leak or noise
A thicker oil can hide symptoms for a while, but it can starve pump inlets at start-up, raise energy use and slow valve response. Find the cause first: worn pump, air entrainment or overheating.
Performance class matters as much as viscosity
If the manual specifies DIN 51524 Part 2 (HLP), choose Servosystem HLP 46 or 68 rather than general anti-wear oil.
Recommended SERVO grades
FAQs
Mixing gives an in-between viscosity and is not recommended as a practice. If it happens with two compatible anti-wear oils, the system will usually run, but switch back to the specified grade at the next change.
Neither is better. They are for different operating temperatures and pump designs. Use the grade the equipment maker specifies.
