On page 12 of the December 2020 G&D it talked about 4 speed transmission oil, of course I was using the wrong one.
What should I be using for oil in the rear end? The rear end has 336 gears and is a Posi Traction.

i use 90 wt gear oil and 8 oz of poi traction lubricant., my favorite being Quell which is becoming more difficult to obtain. it contains oils from a spermaceti whale. mike mccagh

You do want a GL-5 oil in the rear axle. The ring and pinion gear are a hypoid gear set.

You have many options of high end multi-grade that is GL-5 like Rusty said that are superior to what was available in 1966. There are 75w90, 75w110, 75w140 and various combinations around these depending on brand variations.They are all a bit thinner acting when cold but a bit thicker when hot. If it's used for short runs in cooler weather you can go with the lighter fluids. If it gets long runs on hot days or tends to leak, you can go thicker.

You really can't go wrong with any modern name brand oil BUT you still MUST use the GM additive and if Mike above says 8oz, then use the 8oz's.