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ISO 32 vs 46 vs 68 hydraulic oil: which grade do I need?
Short answer: use the grade your pump or machine manufacturer specifies. If the manual gives a range, ISO VG 46 is the default for most indoor industrial hydraulics in the UK, ISO VG 32 for cooler sites and high-speed pumps, and ISO VG 68 for hot ambient temperatures and heavily loaded pumps. The number is the oil’s viscosity at 40 °C — nothing else changes between the grades of the same product.
[YOUR EXPERIENCE HERE — one real example of a customer running the wrong grade and what it did to the pump, or how you decide between 46 and 68 for an African site; then delete this marker]
What do 32, 46 and 68 actually mean?
They are ISO viscosity grades under ISO 3448: the number is the oil’s kinematic viscosity in centistokes (mm²/s) at 40 °C, with each grade allowed ±10% around that midpoint. So an ISO VG 46 oil measures between 41.4 and 50.6 cSt at 40 °C, and each step up the scale is roughly 50% thicker than the one below.
The grade says nothing about quality, additives or approvals. Shell Tellus S2 MX 32, 46 and 68 are the same oil at three thicknesses; the same is true of Mobil DTE 24, 25 and 26 Ultra and Castrol Hyspin AWS 32, 46 and 68.
| Grade | Viscosity at 40 °C (ISO 3448 range) | Typical use | Genuine products we stock |
|---|---|---|---|
| ISO VG 32 | 28.8 – 35.2 cSt | Cooler sites, high-speed low-pressure pumps, machine tools, lifts, precision hydraulics | Shell Tellus S2 MX 32, Mobil DTE 10 Excel 32, Castrol Hyspin AWS 32, Fuchs Renolin ZAF 32B |
| ISO VG 46 | 41.4 – 50.6 cSt | General industrial hydraulics, presses, injection moulding, most mobile plant | Shell Tellus S2 MX 46, Mobil DTE 25 Ultra, Castrol Hyspin AWS 46, Fuchs Renolin ZAF 46B |
| ISO VG 68 | 61.2 – 74.8 cSt | Hot climates, high-load pumps, mining, quarry and construction plant | Shell Tellus S2 MX 68, Mobil DTE 26 Ultra, Castrol Hyspin AWS 68, Fuchs Renolin ZAF 68B |
Should I use ISO 32 or ISO 46 hydraulic oil?
Use 32 where the oil runs cool or the pump is high-speed and low-pressure — machine tools, lifts, indoor systems in an unheated building through winter — and 46 for the general run of presses, injection moulders and power packs at normal operating temperature. If the manual allows either, 46 is the safer default in the UK because it keeps a thicker film as the system warms up.
The practical difference shows at cold start. Mobil’s data sheet for the DTE 20 Ultra series gives a pour point of −36 °C for the VG 32 grade and −33 °C for the VG 46, and a viscosity at 100 °C of 5.8 cSt against 7.1 cSt — the thinner oil moves sooner in the cold and gives up a little film thickness once hot (Mobil DTE 25 Ultra product data).
Should I use ISO 46 or ISO 68 hydraulic oil?
Use 68 where the oil runs hot or the pump is heavily loaded: quarry and mining plant, outdoor equipment in Nigeria, Ghana, Kenya or South Africa, and any system where a 46 grade is thinning out by the afternoon. Stay on 46 for indoor industrial systems at a steady temperature and for any pump whose manual specifies it — a thicker oil than the pump was designed for raises pressure drop, slows response and can starve the pump inlet at start-up.
The same Mobil data shows why: DTE 26 Ultra (VG 68) holds 8.9 cSt at 100 °C against 7.1 cSt for the VG 46, but its pour point is −30 °C rather than −33 °C. More film when hot, less flow when cold.
Can I use 46 instead of 32, or 68 instead of 46?
Only if the equipment manufacturer’s viscosity range covers both grades — and many do, because pump makers publish a permitted viscosity window rather than a single number. Going one grade up is usually tolerated in a warm system; going one grade down in a cold one is the more common mistake, because a thinner oil loses film strength and lets pump wear start.
Never go up or down more than one grade without checking, and never change grade to “fix” a leak or a noisy pump — that treats the symptom. If in doubt, send us the pump model and the ambient temperature and we will check the manufacturer’s range before we quote.
What about HV / HVLP grades for wide temperature swings?
If your equipment starts cold and runs hot — excavators, cranes, agricultural and forestry machines, anything outdoors through a full year — the answer is not to pick a compromise between 32 and 46 but to use a high-viscosity-index oil of the right grade. An HV (DIN 51524 Part 3 HVLP) oil such as Shell Tellus S2 VX 46 or Castrol Hyspin AWH-M 46 thins less as it heats, so it behaves closer to a 46 across the whole range.
Shell’s data sheet for Tellus S2 MX 46 (viscosity index 105) recommends the VX version where equipment sees wide temperature swings (Shell Tellus S2 MX 46 TDS); Castrol quotes a viscosity index above 150 for Hyspin AWH-M 46.
Which grade do I need? A quick rule
Check the pump manual first. If it gives a range and you are in the UK indoors, take 46. If the site is cold or the pump is high-speed, take 32. If the site is hot, the load is heavy or you are shipping to West or East Africa, take 68. If the machine works outdoors through all four seasons, take an HV 46 instead of choosing between grades.
Harlow Lubricants Supplies is an authorised distributor for Shell, Mobil, Castrol, Fuchs, TotalEnergies and Klüber, with 35 years in lubricant supply. Every grade above is genuine, sealed stock in 20 L pails, 205/208 L drums and 1,000 L IBCs, for UK delivery and export.
Browse the full range: hydraulic oil — ISO 32, 46 and 68 from Mobil, Shell, Castrol and Fuchs.