I didn't realize there was such a controversy. Well, you can believe what people post on the internet or you can believe what Jeep published in an actual book. Your choice, but I'll stick with the factory manual.
Conventional 10w-30 is the new spec for the AX-15. I use the cheap stuff from Wal-Mart.
Here's why I use 10W-30 motor oil in my AX-15.
The Mopar gear oil part number used to be 4897622aa, but when you search that part number you'll see it has been superceded by part number 4761839AC which comes up as 10w-30 motor oil. Here's a link to search the Mopar part numbers.
Based on that, I use 10w-30 motor oil. I buy the Wal-Mart "Super Tech" store brand oil because it's cheap, meets all the API (American Petroleum Institute) grade qualifications and I change mine often, at least every 6 to 12 months.
Here's a link to the Jeep Engineers Q&A thread here on JF where I questioned the Jeep factory engineers on this very thing.
I am not saying that some people haven't used 10w-30 and believe their transmission works better. I was simply pointing out to the OP, who is having shifting problems if you'll recall, that he is using non-spec oil.
I posted the factory service manual. All you've posted is other people's opinions. Sorry, but a contrary opinion, even by a Jeep engineer, does not outweigh the official service publication and you shouldn't go telling people that 10w-30 is the new spec in light of such weak evidence. Jeepers come to JF for good information, just as I do. Other people's opinions can be a valuable source of that information. Mean Max may very well be right in that some AX-15's might shift better with 10w-30 (transmissions vary. I tried the lighter Redline MTL in mine and didn't like how it shifted), but until I see an official Jeep publication that says differently, 75w-90 is still the spec.
Okay, so today I decided to pull the transmission. I had to drive the Jeep back to the shop to work on it. I did notice a few things on the way over. I could start the Jeep in gear with the clutch on the floor and I could hold it in place with the brake. If I let off the brake it would inch forward. I was also able to rev match the gears to shift. I also remembered that when it shifted fine, it would grind going into reverse, no matter what I did. (Shift into first, third then reverse didnt help).
The throwout bearing and pressure plate looked good. Pulled the pressure plate and the clutch disc looked good. I stuck my finger in the pilot bearing to check it and when I pulled my finger out, the inner race and half of the needle bearings came out.
The transmission seemed to be stuck to the engine when I pulled it, I would say that it took some extra convincing to get it out.
Is it possible that the problem was the pilot bearing? I am thinking that if it was too tight (Possibly from being damaged during the transmission installation) that it would keep the input shaft of the transmission spinning. Thus, not allowing me to shift gears. It would also explain why I was not able to get it in reverse without grinding.
It's certainly possible, but there is a cause-effect consideration. Did the pilot bearing fail and cause shifting problems, or did something else cause shifting problems which then tweaked the bearing?
To be honest, I agree with your second post and believe it to be a hydraulics issue. Although we're accustomed to seeing the telltale greasy streak on the firewall, master cylinders can fail without physically losing fluid. But since you've already done the hard part and found an obvious problem, I'd replace the bearing, reinstall the tranny and see how it does. If the problem persists then invest in a MC rebuild kit.
What I ended up finding out was that there was a broken bolt on the pressure plate. My helpers were nice enough to remove it and not tell me about it. I wasn't sure if missing one bolt was enough to warp the plate, but I bought a new clutch kit and slapped it in. Bingo, all problems went away. So I cannot say what part was causing the issue, but I have to assume that it was the pressure plate.