Three grades account for most hydraulic oil sold in Southeast Asia: ISO VG 32, 46 and 68. They differ only in viscosity at 40 °C — and yet choosing the wrong one is the most common cause of both overheating and sluggish response in tropical hydraulic systems.
Start with the pump, not the oil
Every hydraulic pump has a minimum inlet viscosity it can survive and a maximum viscosity it can efficiently move. Vane and gear pumps typically want 10–500 mm²/s at operating temperature; piston pumps used in excavators usually prefer 16–36 mm²/s. The oil grade must keep the system inside that window across the whole working temperature range, which in ASEAN means a reservoir that starts at 28 °C in the morning and sits at 75–85 °C by mid-afternoon.
Rule of thumb for tropical mobile hydraulics: if bulk oil temperature regularly exceeds 70 °C and the OEM allows it, VG 46 is the default. Go to VG 68 only when the system runs hot or the pump is worn; go to VG 32 only for cold-start markets or high-speed, low-pressure systems.
What each grade is actually for
- ISO VG 32 — high-speed machine tools, central systems in air-conditioned plants, and mobile equipment in cooler climates. In a tropical excavator it often runs too thin when hot, raising internal leakage and reducing breakout force.
- ISO VG 46 — the workhorse. Most Japanese and Korean excavator OEMs specify it for ambient 20–40 °C. Good balance of efficiency and film strength.
- ISO VG 68 — high-pressure piston pumps, presses, and older or worn systems where internal leakage is already a problem. Costs slightly more in pumping losses.
When viscosity index changes the answer
An AW 46 and an HV 46 have the same 46 cSt at 40 °C but very different behaviour at the ends of the range. HV fluids with a viscosity index of 150+ stay closer to the ideal band when the oil is cold and when it is hot, which matters for equipment that works outdoors across a wide temperature swing — a harbour crane, a logging truck, a machine that starts at dawn in the highlands. The price premium is typically 8–15 %, and it is usually worth it where cold-start cavitation damage is a real risk.
Three checks before you change grades
- The OEM manual. If it names a grade, use it. Warranty disputes are settled with that page, not with a viscosity calculation.
- Measured oil temperature. Not the coolant gauge — fit a thermocouple in the reservoir and log a full shift.
- Filter rating. Moving to a higher-viscosity grade with a 3 µm absolute filter can put the system into bypass on cold start. Check ISO 13357 filterability data.
What we do when a customer is unsure
We ask for the machine model, the OEM recommendation, the measured oil temperature and the current grade. Then we compare it with the base oil and VI of what we would supply, and quote one grade with the reasoning written down. If the answer is "keep what you have", we say that too — a wrong grade change costs more in downtime than it saves in oil price.
For the full technical data of our AW and HV hydraulic range, see the hydraulic oil section or the AW 46 product page.