Went right to the horses mouth,
http://hydraulics.eaton.com/products/pdfs/E-TRLD-TM001-E.pdf
Eatons site lists the factory fill as a premium hydraulic SAE 20W-20 fluid with maximum working oil temp of 82 C and oil viscosity range of 10cST. I'm not positive if I have the model 7 or 11 light duty hydrostatic unit, but both call for 20w20
The site also recommends NOT using a universal tractor fluid:
"Over the recent months, Eaton’s customers have contacted us to consult on issues, which have now
been linked to hydraulic fluid formulation. These issues are specific to yellow metals (brass/bronze)
used in some piston pump and motor components. The most common indication of these problems is
pre-mature degradation of high-pressure, hydraulic system performance and resulting overheating.
In the interest of customer support and education, we feel that it is appropriate to share some conclusions
we have reached regarding these fluids related problems.
- Eaton does not recommend the use of some Universal Tractor Fluids (UTF) or Transmission
Differential Hydraulic Oil (TDH), at this time, because of possible chemical reaction with these
yellow metals. This reaction may be accelerated in the presence of water in the fluid, and/or
high temperature.
- If you are successfully using any of these fluids at this time, we are not recommending a
change. It is however, important to conduct a frequent analysis of the fluid, to monitor
changes in the fluid’s copper(Cu) content, which may indicate a problem.
The industry continues to investigate this problem and as new information becomes available Eaton will
provide the latest updates to our customers."
So, with all this in mind, what 20w20 hydraulic fluid would be best then?