I've been having an issue of losing transmission pressure under load once the machine is fully heated up. The last known oil put in it was Donax Td. The manual calls for the following but I'm afraid the HyGard won't do the trick since the Donax covers J20C spec. Then again, HyGard is a higher quality oil with 900ppm zinc. I had someone reach out to me and say they switched from a 303 hydraulic oil to dextrin for cars and it fixed their issue similar to mine. What kind of oil should I be using for this in the hydrostatic transmission?
I wouldn't compare it to anything car related and use the recommended fluid. If the fluid meets JD J20C spec. losing pressure sounds like it's more an internal leak/problem than the oil used. Did the machine work properly when the current fluid was first put in? Maybe a bad o-ring or something along those lines or an air leak in the suction line. Look closely for an external leak too. How many hours on the machine? Maybe it's just time to r&r some components. If there's a strainer check it and/or cut the filters open and look for debris or metal filings.
Ya was going to use an on spec oil that the manual calls for. I am not sure donax td oil meets the req spec with api gl4. I just know some of these hydrostatic transmission are very picky and the nature of the oil heating up makes me think the viscosity could be the very real issue.
See if the Donax TD oil meets the JD specs. GL4 is too generic. Did the problem just start? Probably need to check some pressures with the oil cold and warmed up. If the oil meets JD20C I don't think it's the problem. The oil is also from Shell so it's a known quality product.