Most tractor hydraulic oil out there meets the JD spec including the stuff at Costco. Gear and hydraulic oil is not interchangeable. Hydraulic oil is typically in the 10/20 weight range equivalent and to the extent additives are used, they are very different that what's put into gear oil. The JD transmissions have tighter tolerances and seals designed for hydraulic oil use. Good way to mess up using the wrong product.
The generics that meet JD spec run between $45-53 up my way per 5 gallons. If you buy by the pail, I highly recommend the pump/hose cover that you can get on Amazon. Mine does 6oz/stroke which makes it easy to count out the proper fill.