I don't find the JD oil that expensive. How cheap do you want to go? I prefer not to take a risk.
But if I was going to take a risk, then probably the cheapest (don't know what is generic) hydro fluid would do. Go for it, pour in the cheapest, and please let us know how it works. If the transmission fails, it will be real difficult to pin it on the hydro fluid, I would suspect. But I doubt it would fail without Deere oil. For sure, not wishing you any bad luck.
Deere has some elaborate oil evaluation laboratories (at least they did in the 50's when I visited their oil lab in Waterloo, IA) and I suspect they are pretty particular about the selection and make-up of their oils. Doesn't mean that other oils cannot be the same, but I won't be finding out. My next 200 hours will have Deere oil for sure. But, I'm a Deere nut. /forums/images/graemlins/grin.gif