ISO VG 68 hydraulic oil is not compatible with all seal materials. Compatibility depends on the seal material, hydraulic fluid chemistry, operating temperature, and the condition of the seals in service.
ISO VG 68 defines only the viscosity of the oil at 40°C. It does not specify base oil type, additive system, or how the fluid interacts with elastomers. Two ISO VG 68 hydraulic oils can behave very differently with seals if their formulation or operating conditions differ.
What ISO VG 68 means and what it does not
ISO VG is an industrial viscosity classification system. ISO VG 68 identifies a viscosity band centred on 68 cSt at 40°C, supporting correct film thickness and hydraulic efficiency when matched to system design and operating temperature.
ISO VG does not define seal compatibility, oxidation stability, zinc content, ash level, base oil chemistry, or suitability for specific elastomers. Seal compatibility must be confirmed independently of viscosity grade.
Why seal compatibility varies
Seal compatibility describes how elastomer materials respond to prolonged exposure to hydraulic fluid. Outcomes include controlled swell, excessive swell, shrinkage, hardening, softening, or loss of mechanical strength. These effects depend on material chemistry and operating conditions.
| Factor | Why it matters | What to confirm |
|---|---|---|
| Seal material | Different elastomers react differently to mineral oils, esters, glycols, and additives. | NBR, HNBR, FKM, PU, PTFE, EPDM, silicone. |
| Fluid type | Mineral oils behave differently from synthetic, water based, or fire resistant fluids. | Mineral anti-wear, high VI, zinc free, biodegradable, fire resistant. |
| Operating temperature | Higher temperature accelerates elastomer ageing and chemical interaction. | Maximum oil temperature and system hot spots. |
| Seal age | Older seals may already be hardened or brittle before a fluid change. | Service history and leakage trend. |
| Fluid condition | Oxidation products and water contamination can indirectly damage seals. | Water content, oxidation stability, cleanliness. |
Compatibility guide for common hydraulic seal materials
This guide applies to typical mineral anti-wear hydraulic oils in ISO VG 68. If the system uses fire resistant, water based, or biodegradable fluids, compatibility must be confirmed against that fluid class.
| Seal material | Common use | Typical compatibility | Notes |
|---|---|---|---|
| NBR (nitrile) | General hydraulic O-rings and seals | Commonly suitable | Temperature limits and compound grade still apply |
| HNBR | Higher temperature hydraulic sealing | Commonly suitable | Improved resistance versus standard NBR |
| FKM | High temperature and aggressive environments | Commonly suitable | Confirm if switching to non-mineral fluids |
| Polyurethane | Dynamic rod and piston seals | Often suitable | Sensitive to water and hydrolysis |
| PTFE | Low friction sealing | Chemically resistant | Performance depends on energiser design |
| EPDM | Water based systems | Typically unsuitable | Not compatible with mineral oils |
When seal issues occur in practice
Seal problems attributed to ISO VG 68 are often caused by fluid chemistry changes, elevated temperature, oxidation, or pre-existing seal ageing. Fluid changes can expose seals that were already near the end of service life.
How to confirm compatibility correctly
Confirm the hydraulic fluid class specified by the equipment manufacturer, identify the seal material, verify operating temperature, then match viscosity grade. ISO VG alone is not sufficient for compatibility decisions.
Where seal material is unknown, equipment documentation or seal suppliers can usually identify material based on part numbers and application.
Related catalogue sections
SINOPEC Lubricant Product Catalogue
Hydraulic oils
Technical Hub
Technical support
For assistance with hydraulic oil selection, viscosity confirmation, or seal compatibility assessment, contact Sinopec Online or call +44 020 8068 3444.