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Hydraulic Oil Classifications and Compatibility
Hydraulic oils are classified by viscosity and additive content. ISO 46 and AW46 refer to oils with a viscosity grade of 46 centistokes at 40°C. The “AW” designation means anti-wear additives are included. SAE 10W motor oil, often used in older Caterpillar machines, has a viscosity roughly equivalent to ISO 32 and is sometimes labeled as Cat HYDO Advanced 10. While these oils may appear interchangeable, mixing them can affect system performance, especially in machines with tight tolerances or sensitive valves.
Terminology Note
In a 1980s-era Caterpillar D4D dozer, the hydraulic tank contents were unknown, and lift cylinders showed signs of leakage. The operator suspected a low-grade oil had been used. While repacking the cylinders was inevitable, the question arose: could ISO AW46 and SAE 10W oils be mixed temporarily to keep the machine running?
The short answer is yes—but with caution.
Recommended Actions and Alternatives
Mixing ISO AW46 and SAE 10W hydraulic oils in older equipment like the Caterpillar D4D is possible for short-term use, but it’s not ideal. Differences in viscosity and additive chemistry can affect performance and seal longevity. For best results, match oil type to manufacturer recommendations or drain and replace with a known compatible fluid. In legacy machines, consistency and cleanliness matter more than brand loyalty.
Hydraulic oils are classified by viscosity and additive content. ISO 46 and AW46 refer to oils with a viscosity grade of 46 centistokes at 40°C. The “AW” designation means anti-wear additives are included. SAE 10W motor oil, often used in older Caterpillar machines, has a viscosity roughly equivalent to ISO 32 and is sometimes labeled as Cat HYDO Advanced 10. While these oils may appear interchangeable, mixing them can affect system performance, especially in machines with tight tolerances or sensitive valves.
Terminology Note
- ISO 46: International Standards Organization viscosity rating; 46 cSt at 40°C.
- AW46: ISO 46 oil with anti-wear additives.
- SAE 10W: Society of Automotive Engineers rating; roughly ISO 32 in hydraulic terms.
- HYDO: Caterpillar’s proprietary hydraulic oil, formulated for extended drain intervals and system cleanliness.
- Gland Nut: A threaded collar that compresses seals around a hydraulic cylinder rod.
In a 1980s-era Caterpillar D4D dozer, the hydraulic tank contents were unknown, and lift cylinders showed signs of leakage. The operator suspected a low-grade oil had been used. While repacking the cylinders was inevitable, the question arose: could ISO AW46 and SAE 10W oils be mixed temporarily to keep the machine running?
The short answer is yes—but with caution.
- Viscosity mismatch: ISO 46 is thicker than SAE 10W (ISO 32). Mixing may result in unpredictable flow rates, especially in cold weather.
- Additive conflict: AW46 contains zinc-based anti-wear agents. If mixed with motor oil containing detergents or friction modifiers, chemical interactions may degrade seals or reduce lubrication.
- System age matters: Older machines with looser tolerances may tolerate mixed oils better than newer equipment with precision hydraulics.
Recommended Actions and Alternatives
- Drain and flush the system if contamination or unknown oil is suspected.
- Use ISO AW32 if SAE 10W is unavailable—viscosity is similar, and AW additives improve wear protection.
- Avoid mixing brands unless compatibility is confirmed.
- Monitor cylinder seals for swelling or cracking after oil changes.
- Label the tank with oil type and change date to prevent future confusion.
- Repack cylinders every 2,000–3,000 hours or when leakage exceeds 10% of stroke volume
- Replace hydraulic filters every 500 hours
- Sample oil annually for viscosity and contamination
- Store hydraulic oil in sealed containers away from moisture
- Use a dedicated funnel and pump for each oil type
Mixing ISO AW46 and SAE 10W hydraulic oils in older equipment like the Caterpillar D4D is possible for short-term use, but it’s not ideal. Differences in viscosity and additive chemistry can affect performance and seal longevity. For best results, match oil type to manufacturer recommendations or drain and replace with a known compatible fluid. In legacy machines, consistency and cleanliness matter more than brand loyalty.
