When you are using your tractor I found with my bx1860, that after about 15 minuets I would pull the filler plug. This in turn let pressure out of the axle and since I started doing this my axle seals have not leaked.
This solved the slow leak I had. After mowing the grass on a warm day a couple months ago I pulled the fill plug and put it back in. The axle hasn't leaked since.