Turbo II transmission. When driving around normally it shifts fine but in hard pulls the 2nd to 3rd shift is a pain and wont let you shift without grinding unless you rev match perfectly. It'll go in just not without fussing and if you want it to go into 3rd at high rpm you'll need to drop the rpm by about 2k to get it in smooth. apart from that the transmission works like a charm, 1st to second is ok and I can slam shift 4th all day long without a fuss (not like I do, just a figure of speech :P)
Ive seen people talk about different remedies for this aside from replacing shifter bushings and synchro's. Most of them saying using redline 75-w90 or a mix of that and ATF fluid but what I found was from 2009 or so, so I was wondering if there was any definitive answer to what you should put in your tranny.
The other thing I've been hearing ever since I put it in the car is a sound in 4th gear. I wont call it a grind or a whine but more of a clattering and only after driving for a while and only in 4th gear. it gets louder if you engine brake. I don't have the insulation piece between the leather piece and the rubber separating the outside from the interior, and I did work at a tranny shop for a while and from what they said transmissions are a lot louder than people think so It may just be that but I was never able to show them the issue so I thought I'd bring that up here as well.

 

I went from whatever was in my previous transmission when I bought the car to Royal Purple (synchromax, I think) and it made a big difference. Still not great, but that transmission clearly hadn't been treated very well.

Redline is probably a good choice too. As long as you're getting a name brand synthetic you should be fine.

If that doesn't fix it then you can inspect the bushings. If those are still good, then things get expensive.

 

+1 change the fluid, i usually end up with redline, but its just easy to get. also if the shifter bushings are bad, it makes the car much nicer to drive when its good, and the boots too, the big rubber boot is expensive, but it makes a big difference. it keeps a lot of heat and noise out.

mine also makes noises in 4th, i think the only fix (if fluid didn't help) is to rebuild it. mine does it at low RPM and high load, so i just avoid that

 

^ this.

... and after reading all the things you described, i tend to think a rebuild would be prudent. things are certainly not going to better, even if the changing to a slipperier fluid quiets the noises and smooths out the roughness.

 

Rebuilding the TII transmissions are a piece of cake. You'll be happy you put new synchros in it. Shifts are smooth and there won't be any clattering. You can also clean out all the boogers that have developed over the decades. If you have the correct sized bearing puller, it makes all the difference. If you just pick up one from O'reillys, it won't pull the front bearing out very easily. You need to be able to lock the puller arms into position like the one pictured in the FSM. If you have something that can do that, you'll be golden. Also, get a nice set of snap ring pliers, with solid tips. The cheapo ones with swappable tips will deform and bend and you will have poor results. These are the two main things that make a world of difference. I did my first one without them and it was a bear. Did the second one with the correct tools and it was quick and easy. All that plus some new shaft seals and bearings, and you'll be good to go.