Okla State Univ (OSU) was originally Okla A&M just like in that other state to the south.
But back to filters:
Check the obvious...
+Confirm filter has only ONE gasket in place.
+One gasket oil-wetted not dry when installed.
+Let machine warm up before operating---warms hyd oil (machine in neutral, clutch 'out' not locked down). 10*F might wish at least 15-min medium idle warm up period, longer better.
+Filter tightness when installed---1.5-turns past hand-seating is what I use.
Then think about pressure relief valve setting and what might have transpired to change it.
Which leads to possibility of water in system trapped by filter and / or affecting relief valve. How long since last change? Are the one/two breather goosenecks present or missing allowing moisture entry?
Is the leak actually the filter itself or might it be the mount or a pipe or hose leading to the mount?
You mention hydraulic oils by ISO but don't say if you're using Kubota SUDT(2) or what.
I'll here start an argument (please be gentle fellow forum members) by saying not all oils are equal and HSTs can be pretty sensitive to their fluids. It could be--could be--your oil is somehow interacting with the filter gasket. Low probability, but possible, depending on what your oil actually is and what its intended application is---and for that matter depending on the filter [gasket] brand itself. If you can't otherwise find a solution consider trying premium Kubota fluid and filter, even at the additional expense.
Please post back your experiences so we may all learn.