I would also like to say something about the thicker oils. Be very careful!!! Notice the 98 Amigo with the manual tranny that takes Dextron III. There is a reason for that. The reason is that when computers now design and manufacture our parts tolerences are much tighter and more precision between the gears, synchros, countershaft bearings, etc, that normal thicker oil will have a difficult time getting BETWEEN all those parts, shaft surfaces and needle roller bearings and lubricating them properly. New engines take 5w-30 or 0w-30 because the bearing tolerences are so much tighter as compared to yesteryear, any thicker oil will not allow proper lubrication. Thinner oils do possess the additives to lubricate just as well and even better than thicker oils due to the fact that they get "in and out" of an area quicker, thus dispersing heat and reducing friction more efficiantly. Since gearboxes and differentials "splash" their oil around, thinner gear oil,(75W-90), with a good hypoid additive will always produce superior lubricating abilities over say 90W-140, which doesn't splash half as good. Yes, there are specific applications which will use 140, but we're talking isuzu trucks and the like.
Remember also, my 86 trooper takes 140 gear oil if temps are over 90 degrees constantly and 80W-90 for all other applications. But that was 16 years ago. 75W-90 gear oil of today is far superior in its additive package and lubricating abilities than any of yesteryears 140 gear oil ever was. So I use 75W-90.
Today's synthetic gear oils, if they could speak, would laugh at our trucks and the ease of which they could protect them!