I have found a square plug on my tranny on the passenger side, about midway up on the tranny housing. Is this the fluid level/fill hole? If so, what should I add if needed? Any particular type gear oil? Thanks in advance. Tom. :)

That is indeed the fill/level plug.The recommendations of those in the know say to use only an API GL/4 rated gear oil such as the sta-lube 85w90 available from Napa, part #SL24239 is for a 1 gallon jug but it should also be available in quarts.

Great! Thanks. Tom.

You fill gear oil in that hole until it starts to weep out. Do not over fill.