I chose to use JD hygard in mine since is small volume of oil and have JD dealers in every town around here. I am not a fan of using generic oils on hydrostatic drives. They may be fine but for four gallons of oil is savings enough??
Would also recommend using factory filter since once is a suction filter and there have been some reports of aftermarket causing pump to starve for oil.
I'd say the same thing. There are no standards for trans/hydraulic oil like there is for engine oil.
It makes sense to use a name brand oil. JD HyGard is reasonably priced and available everywhere.
The hydraulic filter is a suction type, so best stay with the factory filter.
BTW, since the hydraulics still worked I doubt you did any damage. As others have said, that system holds gallons of fluid. I think my Kubota holds somewhere around 12 gallons, and only the last gallon or two shows on the dipstick.
When a hydraulic system begins to starve for oil all the hydraulic unctions quit working and the noise is very loud - you wouldn't miss hearing it.
I like to change all the engne, trans/hydraulic fluid (which is also the rear axle fluid in tractors)) and the front axle oils along with all the filters. I do that for sure in the first 50 to 100 hours of operation and that is when I put in my top quality oils and filters. Doing that at low hours gets rid of all the manufacturing swarf and dirt.
And then I go to standard maintenance intervals or longer. If the tractor is used for light occasional chore work, and I have used top quality synthetic oil and OEM filters I will extend the intervals up to double.
However, I will shorten the intervals if the work is heavy, or high RPM, or if the machine smells like it has been running hot.
I personally buy the best oils because it seems like cheap insurance. But those real good fluids are costly - especially all those gallons of JD HyGard trans/hydraulic fluid. And honestly I must say in my 50+ years as a mechanic I have never seen a case of engine/tranny wear or failure that I could definitely blame on poor quality oil - as long as there was enough of it and it was roughly the right type oil.
....And I've drained some pretty funky oil - particularly out of old machines back when before detergent oils became the standard. Some I'm sure had never been changed -only added to - and even on hose the moving parts were fine.
I have seen wear and failure due to water getting into the oil - especially the trans/hydraulic oil which seems to attract water and turn milky and rust parts. And I've also seen failures due to poor quality filters as well as due to air leaks in hydraulic suction lines feeding the hydraulic pump.
Mostly it is just common sense.
rScotty