it's always been my understanding:
it is not the hydraulic system that is the issue and requires the GL-1 oil.
It's the yellow metal bushings and bearings in the transmission that you are trying to protect using the mineral oil.
but, since these old tractors are not used hard like they were 50 years ago, you should get by with the car motor oil.
however, as long as I can still buy GL-1 oil for my old 1957 TO-35 tractor, I will use the mineral oil.
The lift still works good and the gears still shift fine in my old tractor.
That oil you bought, Shell rotella 15w40, is great oil, I use it all the time in my dodge diesel truck and also use it in the motor of my old fergy tractor as it still has the zinc additive to protect the valves.