Bush hog gearboxes change the drive angle from the input shaft to the cutting blades. A typical gearbox houses gears, bearings and seals. Each of the components requires the proper amount and type of gear oil to operate for extended periods. Most gearboxes have a gear oil fill/check plug only. However, this does not prevent the ability to change the oil on occasion.
Use a Hand Pump
A hand pump with a long plastic tube on the end will remove the old gearbox oil. Hand pumps for this purpose are available at most automotive parts houses. They are typically used for servicing transmissions and power steering pump reservoirs on automobiles. A hand pump works nicely on a bush hog gearbox when the oil is warm. Immediately after using the bush hog, shut it down and remove the fill/check plug. Snake the plastic tubing into the hole to the bottom of the gearbox. Pull out on the pump handle to create suction, and draw the old oil out of the box.
Tip the Bush Hog on Its Side
A bush hog with a cutting diameter of 5 feet or less can tip on its side. The gearbox fill/check plug typically mounts on the side, or at an angle, into the gearbox. Tipping the deck on its side allows the oil to drain. Warm the oil up by either operating the bush hog or setting it in direct sunlight. With the bush hog disconnected from the tractor, tip it on its side, and remove the fill/check plug. Catch the oil in a bucket, and set the bush hog back down.
Remove the Gearbox
Removing the gearbox from some models of bush hogs is not as difficult as it sounds. Decouple the drive shaft from the input of the gear box. Remove the mounting bolts securing the box to the deck of the bush hog. A spline shaft connects the gearbox to the cutting wheel. With the bolts removed, lift the gearbox off the bush hog deck, and turn it on its side over a bucket to drain the oil.