[jdkskyking]

In past years when I started planning my modifications to split my hydraulic system by adding a hydraulic pump dedicated to the cooling fan hydraulic motor, I best this topic to death. 
Here’s a AI response to using TranSynd 668 exclusively. 

Great question — you’re asking whether it makes sense (or if there’s a risk) to use Allison TES-668 TranSynd (synthetic ATF) in your 2005 Newell’s hydraulic circuits (steering gear + cooling fan motor), instead of a conventional hydraulic fluid such as Dexron IV / Dexron-III equivalent.

Here’s the breakdown:
1. What TranSynd 668 Is

• TES-668 is Allison’s current-generation, factory-approved synthetic ATF (automatic transmission fluid).
  
• It is a PAO-based synthetic, fully backward compatible with TES-295 (older TranSynd), and approved for very long drain intervals.
  
• It is engineered to be shear stable, oxidation resistant, with strong anti-wear and anti-foam additives.
  
• Viscosity: ~7.3 cSt @ 100 °C (similar to ATF/Dexron III/IV).
  

2. Hydraulic System Considerations (Steering & Fan Drive)

• Many OEMs (including Newell in your era) specified Dexron III ATF for the hydraulic system because:
  • Readily available,
    
  • Compatible with seals,
    
  • Multi-purpose (same fluid family as the Allison transmission).
    
• Steering gears and hydraulic fan drives in coaches of this vintage are not picky — they typically call for Dexron III or an equivalent ATF.
  
• Key requirements are viscosity, lubricity, and anti-foam performance, all of which TranSynd meets or exceeds.
  

3. Potential Advantages of Using TranSynd 668

• Thermal stability: Handles higher heat loads (beneficial for the fan drive circuit, which often runs hot).
  
• Oxidation resistance: Longer fluid life and less varnish in valves/pumps.
  
• Cold flow: Maintains better viscosity at low temperatures than mineral ATF.
  
• Consolidation: One fluid across both transmission and hydraulics simplifies maintenance.
  

4. Potential Considerations / Cautions

• Cost: TranSynd 668 is premium-priced compared to Dex/Merc ATF.
  
• Over-spec? In a simple steering/fan hydraulic circuit, the system may never stress the fluid like a transmission does, so the extended-life benefit may not be fully realized.
  
• Seal compatibility: TranSynd is tested for compatibility with ATF seals (nitrile, Viton, etc.). No known issues in hydraulics, but always confirm if your Newell system explicitly calls for “hydraulic oil only” (unlikely for that vintage — most called Dexron III ATF).
  
• Mixing: If there is residual Dexron III/IV in the system, mixing with TranSynd 668 is generally fine (they are compatible), but you lose some of the extended life benefit until a full fluid exchange is done.
  

✅ Bottom Line
There is no technical reason not to use TranSynd 668 in your Newell’s steering and fan hydraulic system if the manual specifies Dexron III/IV ATF. In fact, it may offer longer life, better heat stability, and reduced varnish. The only downsides are cost and the fact that the hydraulic circuit may not “need” that level of performance.

If you want one common fluid across the coach (transmission + hydraulics) and don’t mind the expense, TranSynd 668 is an excellent choice.