Transmission Oil Choices for the Lull 644 Highlander II

[MikePhua]

The Lull 644 Highlander II and Its Mechanical Lineage
The Lull 644 Highlander II is a telescopic material handler developed for rugged jobsite conditions, combining lifting power with precise boom control. Originally manufactured under the Lull brand—later acquired by JLG Industries—the Highlander series gained popularity in North America during the 1990s and early 2000s. With a rated lift capacity of 6,000 lbs and a maximum reach of 42 feet, the 644 Highlander II was designed for framing, masonry, and industrial maintenance.
Its drivetrain typically includes a John Deere diesel engine paired with a ZF off-highway transmission. This transmission system is engineered for high torque output and smooth directional changes, but its performance depends heavily on correct fluid selection and maintenance.
Understanding Transmission Oil Requirements
Transmission oil in the 644 Highlander II serves multiple functions:

• Lubricates gears and bearings
  
• Transfers hydraulic pressure for clutch engagement
  
• Cools internal components
  
• Prevents corrosion and varnish buildup
  

The transmission dipstick on many units is labeled “C3 transmission fluid,” referring to a specification originally developed by Caterpillar for powershift transmissions. C3 fluids are designed to maintain viscosity under high load and temperature, resist foaming, and provide frictional characteristics suitable for wet clutch packs.
C3 Transmission Fluid and Modern Equivalents
While Caterpillar C3 fluid is no longer widely marketed under that name, many modern oils meet or exceed its performance criteria. These include:

• Universal Tractor Transmission Oil (UTTO)
  
• JD Hy-Gard (John Deere)
  
• Mobilfluid 424
  
• NAPA Premium Tractor Hydraulic Fluid (C3-rated)
  

These fluids typically offer:

• Viscosity range of SAE 10W-30 or 15W-40
  
• Friction modifiers for clutch engagement
  
• Anti-wear additives for gear protection
  
• Compatibility with wet brakes and hydraulic systems
  

In one field case from Alberta, a contractor used JD Hy-Gard in his Highlander II and reported smoother gear shifts and reduced transmission noise during cold starts.
Temperature Considerations and Seasonal Adjustments
One challenge with transmission oil selection is ambient temperature range. A single oil grade may not perform optimally across extreme seasonal shifts. For example:

• SAE 10W-30: Suitable for cold climates, faster circulation at startup
  
• SAE 15W-40: Better for high-load summer conditions, improved film strength
  

Operators in northern regions often switch to lighter oils in winter to prevent sluggish response. In contrast, desert crews prefer heavier grades to maintain viscosity under heat stress.
Wet Brake Fluid and Hub Maintenance
The Highlander II also features wet disc brakes, which require specific fluid to maintain friction and cooling. According to service documentation:

• JD Hy-Gard: 2.4 pints per hub
  
• Friction modifier: Add 3 oz per hub
  

Using incorrect fluid in brake hubs can lead to noise, reduced braking efficiency, and premature wear. A technician in Texas once discovered that his noisy brakes were due to standard hydraulic fluid lacking friction modifiers. After switching to Hy-Gard and adding the recommended additive, the noise disappeared and braking improved.
Transmission Service and Fluid Checks
Routine transmission maintenance includes:

• Checking fluid level with engine idling
  
• Inspecting for leaks around seals and fittings
  
• Replacing fluid every 1,000 hours or annually
  
• Cleaning or replacing transmission filters
  
• Monitoring for discoloration or burnt smell
  

After servicing the transmission, always recheck fluid level with the engine running at idle. Add oil as needed to reach the “FULL” mark on the dipstick.
In one case from British Columbia, a fleet manager noticed erratic shifting in a Highlander II. After draining the transmission, he found sludge and metallic particles—signs of overdue service. A full flush and filter replacement restored normal operation.
Sourcing Fluids and Avoiding Mislabeling
Operators often struggle to find C3-rated fluids due to outdated labeling. When purchasing:

• Look for “Caterpillar TO-2” or “C3 equivalent” on the label
  
• Confirm compatibility with wet brakes and powershift transmissions
  
• Avoid generic motor oils unless explicitly approved
  

Some suppliers offer bulk fluids with custom labeling. Always verify specifications before use. In one instance, a mislabeled drum led to clutch slippage and a costly rebuild.
Conclusion
Choosing the right transmission oil for the Lull 644 Highlander II is essential for drivetrain longevity and performance. While C3 fluid remains the baseline, modern equivalents offer improved protection and seasonal flexibility. By understanding fluid roles, monitoring service intervals, and adapting to climate conditions, operators can ensure smooth operation and extend the life of their machines. Whether lifting trusses in Montana or moving pallets in Florida, the right oil keeps the Highlander II shifting strong.