Given news wrote:

Have you checked Amazon, NAPA, or farm supply stores like Tractor
Supply,
Rural King, or Orscheln? The first one that pops up at Amazon is $35.

I admit I'm running off of fear since all the Toyota guys say that you must
use either Amsoil or Royal Purple or Redline GL4 75W90 in the manual
transmission.

Even the Royal Purple 75W90 gear oil is $60 per gallon.
https://skspeed.com/royal-purple-013...-gl-5-1-quart/

All I want to know is what's so special about these gear oils that they
cost $60 to $70 a gallon (where I need about a gallon)?

Looking up the gear oils, I find technical information but nothing answers
my question which is why I asked here.

A study of automotive gear lubes
http://www.technilube.com/brochures/...hite_paper.pdf