That's common to have old hydraulic units call for engine oil for use since back then there were very few options for dedicated hydraulics, a lot of them used the trans and/or rear end for the hydraulic reservoir. Even now though, the AW hydraulic fluids for tractors are more of a recommendation for a range of temps rather than a single use fluid like engine oils are now. If you want more flow at colder temp switch it out to a lower AW fluid like AW32 or 46. For an extreme hot weather example, if you are out in 110F temps running a skid steer with a hydraulic powered brush cutter you will need AW100. I run AW 32 in my old Dear John since its a 1939 tractor and it only goes up/down/tilt and I dont care about slow pressure loss during summer use.
And I laughed when I saw your new grill! It reminded me of:
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