Bird
Rest in Peace
This topic was discussed on TBN about 7 years ago.
And with about the same conclusions. At the time, the manual for my 1999 B2710 had the same recommendation. I had changed it to the gear oil because that's what I was accustomed to for differentials and manual transmissions. It seems that some think the "heavier" oil provides a tougher better lubrication, but then others believed the "thinner" oil would get into tiny clearances that the "thicker" oil would not. Anyway, since the manual recommended either/or and the tractor came originallly with UDT in it, I later changed back to UDT in the front axle.
And of course I still don't really know whether one is actually any better than the other. But I suspect you'll be alright with either as long as you have the proper amount in there and don't get water in it.