Trans Fluid....+4?

My 1971 Valiant, 225/ A904 trans.
I am going to do a fluid change, and also band adjust.
It sat for 15 years, and has only 68k on it.
I don't know if it was ever changed "I don't think it was", but the fluid is clean, and I want to do some maint.
I was told to use Dex3/Merc at Advance.
I was also told to use ATF+4.
I am being told two different things by lots of folks, I am getting confused.

I see the Castrol ATF+4, but it is synthetic. And not cheap!
I was at WalMart, and saw the ATF+4 and took a photo of both sides of the bottle so you guys could see the specs on their Super Tech Fluid.
It is Half THE PRICE! But does not say synthetic. I am lost. Is this okay? I am so lost on what trans fluid to use.