Look in the manual if you have one, the spec on the oil is either a J20A or J20B or J20C, etc. Match that and you should be fine. Or you could go with John Deere Hy-gard and there are definitely people who swear by OEM fluids. If it is a leaky old machine (I have a 1974 JD 410b) you could go with the cheaper oil........... or not.
At a local farm/ranch supply place 5 gal of the J20 spec is $30-$40, JD Hy-gard is about double that.
There is a lighter viscosity for colder climates if you use it much in the winter.
By the way, the transmission oil is the same as the hydraulic oil on that machine. (maybe you knew that?