Its been theorized that the lower seals in the front axles are more prone to leaking when used heavily and filled with tractor fluid vs gear oil. I personally think that a good gear oil is much better in that application, as it does not mix with the rest of the tractor and is a dedicated fluid for a dedicated purpose.
When I took delivery of my Kubota L2501, I checked all the fluids and found the front axle to be low. I added gear oil to it. At the 400 hr service, I changed to 75w-90. Now over 500 hours of heavy use (loader often maxed out, difficult terrain, lots of loader cycles) I haven't had any issues.