Given , Stormin' Norman
wrote:

The specific products you mentioned above are all synthetic lubricants
and cost more to manufacture, hence, they are more expensive at retail
than their non-synthetic counterparts.

I think I have the answer as to "why" now, which is that the synthetics
cost about $70 per gallon while the not synthetics cost about half that, at
around $35/gallon.

But why, when motor oil is commonly half that still, is gear oil at even
$35 per gallon, but much more importantly, what would be a reason to choose
between the $35/gallon SAE 75W90 GL-4 gear oil versus the $70/gallon stuff.

Is it all due to synthetics?
Are synthetics better for a two-decade old 5-speed manual transmission?

The reason I'm looking is that the shifting is getting harder to get into
gear lately, so someone suggested putting the high end transmission fluid.