Hydraulic Oil Selection and System Cleaning for the Caterpillar 955L Track Loader

[MikePhua]

For the Caterpillar 955L hydraulic system, the recommended oil is SAE 10W mineral-based hydraulic fluid or AW32. In warmer climates, AW46 or even AW68 may be suitable, but viscosity must match operating conditions. If water contamination is present, the system must be flushed thoroughly before refilling.
Caterpillar 955L background and hydraulic system design
The Caterpillar 955L was introduced in the late 1970s as part of the 955 series of track loaders. Built for earthmoving, demolition, and utility work, the 955L featured:

• A turbocharged 3304 four-cylinder diesel engine
  
• Powershift transmission with torque converter
  
• Open-center hydraulic system with gear pump
  
• Hydraulic tank with side access panel
  
• Mechanical linkages for bucket and lift control
  

Caterpillar, founded in 1925, has long standardized its hydraulic systems around mineral-based oils. The 955L was widely sold across North America, Europe, and the Middle East, with thousands of units still in operation today.
Hydraulic oil types and climate considerations
The original service manual for the 955L specifies SAE 10W mineral oil for hydraulic use. However, modern equivalents include:

• AW32: Suitable for cooler climates or general use
  
• AW46: Preferred in moderate to warm conditions
  
• AW68: Used in hot environments where oil thinning is a concern
  

Shell Tellus 46 and 68 are zinc-based anti-wear hydraulic oils that meet ISO standards and are compatible with older Caterpillar systems. Zinc additives help reduce pump wear and extend seal life.
Water contamination and system cleaning
If the hydraulic oil appears milky or foamy, water contamination is likely. This can occur due to:

• Condensation inside the tank
  
• Leaking cooler or breather cap
  
• Improper storage of refill oil
  

To clean the system:

• Drain all hydraulic oil from the tank and lines
  
• Remove the side access cover from the tank
  
• Flush the tank with diesel fuel or a compatible cleaning agent
  
• Wipe down internal surfaces with lint-free cloths
  
• Replace filters and inspect suction screen
  
• Refill with fresh hydraulic oil matching climate and spec
  

Avoid using detergents or water-based cleaners, as they may leave residues that degrade seals or emulsify with new oil.
Preventive maintenance and inspection tips

• Check hydraulic fluid level weekly
  
• Inspect breather cap and seals monthly
  
• Replace hydraulic filters every 500 hours
  
• Sample oil annually for water and particulate contamination
  
• Store refill oil in sealed containers away from moisture
  

One operator in Greece noted that his 955L performed better with Shell Tellus 46 than with older 10W oil, especially during summer grading work. After flushing the tank and switching oils, lift speed improved and pump noise decreased.
Conclusion
The Caterpillar 955L hydraulic system performs best with SAE 10W or AW32 mineral oil, though AW46 or AW68 may be used in warmer climates. Zinc-based oils like Shell Tellus are compatible and offer enhanced wear protection. If water contamination is present, the system must be flushed thoroughly before refilling. With proper oil selection and maintenance, the 955L remains a reliable workhorse decades after its release.