I have a 60-series five speed and have drained the old gear oil out of the transmission. I assume the square-headed plug on the left side of the transmission, halfway up and where the bellhousing meets the transmission body, is the fill plug. I'm asking, though, because I simply cannot shift it to open it and refill the transmission. I'm putting enough leverage on it I'm afraid I'm going to round it off. I'm using a 12 point socket. Is this the right plug? Is there something else I could open and pour in about 1.2 liters of gear oil if I can't get the plug loose?
Thanks for any advice.

You can get the plug loose. It is a square head plug, 14mm.

I use a pipe wrench, that works a treat.