Rear Axle Questions...

I was under my 71 Valiant today, "still new to me".
I am going to plan on changing my rear end oil.

What weight should I use? slant six, automatic.

Also,,,what size is that odd shaped torx looking nut?

And,,I do see some oil leakage around where the driveshaft hooks onto the rear axle. Is there a seal there I can replace? Theres nothing on the floor, but it looks wet around it.