Boys we are talking about gearbox oil.Now pay attention or go sit in the corner.

That same kinda MFGs that can't make up their minds about taking caps off batteries while charging ?
Or to do something, untill the factory bulletin comes out telling you not to ?

I wonder what changes in internal materials/machining processes, etc, were made in the Ford transmissions that were filled with 80/90 wt. oil for decades that are now running on 5 weight. If there are no engineering changes internally that Allow the lower weight oil, then the previously dictated oil was wasteful of millions of gallons of gasoline. How many manufacturers recommend an oil in a given application because it has become "industry Standard"? How much unwillingness to change is out of fear of public perception?
Given the  clearly demonstrated failures in this post of manufacturers to be always correct in other areas, it would seem a bit odd to just say "Well, sure, theyve made mistakes but when it comes to deciding which weight oil to use, they know what theyre doing".

Every report Ive read about why engines are not worn out at 100,000 miles now is almost all due to improved lubricants. Some minor improvements in metallurgy, but the overwhelming majority is attributed to the improvements in oil. Maybe todays 5wt is simply good enough to handle what used to require an 80 wt?