Given ,
wrote:

You are looking at a high end synthetic oil. GL4 is getting a bit
harder to find, but lots of GL5 for uder $25 a gallon in the USA - a
bit more in Canada. Walmart USA carries LubKing GL4 90 weight for $32
a gallon.

I'm told to get either Amsoil or Redline GL4 because it's for a Toyota W59
transmission with yellow metal that can be stripped off by GL5 additives
(plus there are no hypoid gears in the manual transmission).

So it's going to be GL-4 but which one?

Even the Amsoil is $70 per gallon.
https://www.amsoil.com/shop/by-produ...r-lube-75w-90/

What's so special about these two fluids that makes them "worth" 70 dollars
a gallon?