Anyone can weigh the risk of using something else, and probably come out just fine.
I just use the Deere oil, as it isn't much more expensive (I don't know if it is at all). In 5 years I have changed the transmission oil once. Have you calculated what you would save by buying something else?
In my manual, it says Deere Hy-gard I believe, or its equivalent (and then gives the specs of the equivalent).