Len S. This is from the Sauer Sunstrand manual on the 15 series pump. As for characteristics of the fluid selected should include: Viscosity, Oxidation, Thermal Stability, Shear Stability, Low Temperature Fluidity, Rust Inhibition, Anti Foam, Water Compability, Filtration Limitations, Anti-Wear Properties.
Fluids used successfully in the hydrostatic transmission are: (1) anti-wear hydraulic oil, (2) automatic transmission fluid - Type F. (3) hydraulic transmission fluid (type used by the Agricultural Industry for combined hydraulic, transmission, and wet brake systems.
The selected fluid should provide a minimum viscosity of 47 SUS at 210 degrees Fahrenheit. A maximum measured viscosity of 6000 SUS at the lowest expected startup temperature.
Hydraulic transmission oil has a 0 Degrees F viscosity SUS of 12000. At 100 Degrees F... 233. At 210 degrees F a viscosity SUS of 49. A viscosity index of 100. Pour Point at zero degrees Fahrenheit of -35 and an operating range (typical 0 degrees F) of ... 0-200 degrees Fahrenheit.
This should give you something to compare your 30 weight engine oil against. One more thing. Maybe (not saying that a mistake was made) ... maybe, just maybe a misprint was made in stating that engine oil could be used in a hydrostatic transmission. The party typing up the manual was still thinking about the gear drive transmission. Also, would you put transmission oil in your engine? Just a thought.