Originally Posted By: cronk
Hygard is a hydraulic/transmission fluid. Otherwise known as "tractor fluid", it will function as hydraulic fluid and transmission/differential lubricant.
It comes in regular or low viscosity flavors.
Low viscosity is the recommendation for most of the garden tractor hydrostats. I'm not familiar with the OP's specific transaxle, if it was spec'ed by Deere for Hygard, I'm sure that would be a good fluid to use. Amsoil also makes a synthetic all season tractor fluid I may try in my JD 317 hydro.
If Tuff Torq says 15w50 or similar motor oil is recommended, then the Mobil1 would be a good choice.
The viscosity difference between the JD low vis and a 50 weight engine oil is quite a spread.
If you have a severely worn or internally leaky pump causing a noise, a high viscosity fluid may temporarily help, but if you have to use a high vis fluid in a system designed for a low vis fluid, then you have a problem.
Who was the tech you spoke to?