Given , Ed Pawlowski wrote:

Synthetics last longer and lubricate (reduce friction) better. Most of
us don't need it but if you are planning yo keep the vehicle for a long
time, use it under harsh conditions, it is worth the extra money.

Corvettes come from the factory with synthetic engine oil. The Cruze
does not.

I didn't realize these are "synthetics" as compared to whatever "normal"
gear oil is. Is that what's so special about what everyone seems to
recommend on the Toyota forum?

They all recommend mostly Redline, but sometimes Amsoil and less often
Royal Purple, all of which are over $60 and closer to $70 per gallon.

Looking up what I'm supposed to use, I found this paper which said not to
use GL5 and yet the Royal Purple is GL4/GL5 which this paper says is GL5.
http://www.widman.biz/uploads/Transaxle_oil.pdf

All I'm looking for is information to make a decision as to what to put in
a very old (decades) W59 Toyota 5-speed which has synchronizers and yellow
metal.