-I had the same problem and determined that it had Loctite on it from the factory. Put a small propane torch on the plug area to warm it, and it came out without any damage. There are 2 plugs on the gearbox and a vent at the top. The bottom plug is for draining, the plug in the front middle is the gear oil level, and you take the top vent off to fill the gearbox. Everyone that I have drained had water at the bottom. When using the unit, it gets hot and it will pull in some moisture. Eventually, it will have enough to turn the gear oil to a tan color. Drain and refill every year and it is cheap insurance.
Next, put some chassis grease into the auger shaft to keep that lubricated. You will find a grease fitting on each auger. Then check the shear pins to make sure that they are correct and not just some bolt someone installed when they ran out of shear pins. I have seen shear pins bent because someone put the lock washer under the head of the bolt. Last thing, make sure that the augers are properly clocked. There is a label on the inside of the auger area with a correct diagram.