Rogbonb, I am not 100% certain, but experience tells me that you have a bad bearing (at least one) in your gearbox. The gearbox translates horizontal energy (PTO shaft) to vertical energy (bush hog blade shaft). Typically, inside the gearbox there are two gears that are beveled at 45degrees and mate together. This is what changes the direction of the energy. Each beveled gear runs on its own shaft. These shafts have rolling element bearings (ball bearings?) on their ends. I suspect that one of these bearings has failed. When this happens heat is generated by the failed parts of the bearing running against one another with an abnormally high loading.
You said that it takes about 2 hours for the oil to overflow. That is the period of time that it takes to get the parts hot enough to "flash off" the lube oil. Even changing to a higher viscosity oil will only slow the process a little, but the flashing is what you see coming out of the gearbox. You will have to take the box apart and repair the bad elements. Like I said, I am not 100% sure, but the symptoms point me in that direction. Let us know what you find to be the problem and good luck. I know rebuilding a gearbox would not be on the top of my "want to do" list.
