Front axle has a site glass on the left rear of the axle.
I recently "Tripped over" a John Deere Tech. Service Bulletin on this topic while searching for some other information.
According to the bulletin, The use of UTTO (Universal Tractor Transmission Oil) IMPROVES the life of the seals substantially, and the bearings as well.
All without troubles with the gears.
Win Win!
And if it's from J-D, You Know it's good ;-)