You can't just add to it anyway, you'll end up with gear oil pouring out of both ends of the transmission.... BAD.
The drain plug is on the bottom and the fill plug is on the side just below the level of the BOTTOM of the drive shaft's input shaft. The fill plug it located at the "full" level so you cannot over fill the transmission. Whether you use this to fill it or decide to add from above via the shifter housing opening the fill plug so excess will drain will get you to the right amount.
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