Ron, with a few caveats, I'd recommend that you use what your manual is calling for, namely 90w. Thicker is not always better. In fact, in my International trucks, I'm switching from 80W-90 gear oil to straight 50W motor oil in my manual (synchro) transmission. Interestingly, this is NOT what the manufacturer originally recommended, but I have it on very good authority that my tranny will work much better with the 50W motor oil.
It seems that you can't always go by what the manufacturer recommends, although I think that the vast majority of the time it's advisable. Another case of going against the manufacturer's recommendations might be motor oil in a Kubota. They recommend 10W-30 for the most common temperature ranges, but it's very hard to find 10W-30 in a diesel formulation, whether conventional or synthetic.
As for [90W not being as thick as it used to be] ... that makes very little sense to me, and I'd disregard that input.
If your tranny is non-synchro, I'd definitely go with the 90W, or 80W-90, or 85W-90, with whatever EP rating the manufacturer recommends. If it's a synchro tranny, you might want to do some more research to see if you really need EP additives. While EP additives are usually needed in differentials, they are often not needed (or desirable) in synchro trannies. (I'm told they can interfere with the ability of the synchro rings to engage the gears that they're supposed to get up to speed). Maybe a GL-1 rating (no EP additives) in a 90W would be a good choice in a synchro, or a straight motor oil (30W, 40W or 50W).
As you can tell, I don't have a definitive answer for you, and won't pretend to. Just wanted to give you some food for thought. But again, I don't think that 90W-140 would be such a good idea.
Good luck, John
