Replaced axle seal. Filled with dealer recommended JD Hydraulic oil. Ran for 1hr and now will not climb small hill with or without mower running. Engine RPM up, no power to climb. Internal oil reservoir filled to specs.
My guess is that you have air in the hydro that needs to be bled out and then the hydro will need topping up.
You could talk to a Deere mechanic at the dealership. He may tell you how to bleed it. Another option is to determine the make and model of hydro you have and then go to their website. Many of them have downloadable manuals. HydroGear, TuffTork etc.
Here is a link to the K46 transaxle used in most x300 mowers. At the bottom of the specs it looks to use 10w30 CD engine oil 1.9 liters. I recall some using the synthetic oils too when rebuilding.