I would just bite the bullet and buy the NH filters and oil. I know that's easy to say when it's not my money, but unless you are a large outfit running several machines with multiple changes a year, the savings isn't worth it to me vs the risk.
A generic fluid might be fine, or you may find you have to dump it and buy the factory fluid anyway due to decreased performance or another issue. I know NH doesn't make their own filters, but the hydraulic ones seem to be pretty special stuff, not sure a Wix that is meant to cover multiple different use cases is what I would want. And how much is saved over how many years of runtime?
I guess it comes down to elimination of variables for me. If I were to use factory fluids and filters, those parts shouldn't contribute to a problem, and would help me troubleshoot the other items better.
One other question... Do you use your tractor with any implements that connect to hydraulic outlets? If they sit out in the weather there is a potential for some cross contamination or water intrusion there. But it also is very possible it's just condensation that has accumulated over time.