gmcjr,
  Yes, that chart does help.  It helps a lot.  The only thing is, now you've embarrassed me because when Carl sees this I'll be in trouble.  I just hate it when he's right.  (Just kidding.) 
 
  Seriously, though, this chart has caused me to do a Google study of viscosity classifications (in order to properly understand the chart).  I must admit that it's not the first time I assumed something to be a certain way just to find out it wasn't that way at all.  I remember my dad quoting an old German friend, "So soon vee get old. So late vee get smart."

  So, my conclusion will be, as I go though my Jeep preparation for this summer's drives, I will inspect the transmission gear oil for metalic flecks.  If there are any I will drain it and install straight mineral oil, most likely aviation 50 wt. non-detergent, uncompounded engine oil.

  I have saved the chart where it will be available for easy access in the future.  Thanks again, I really do appreciate the "education".

Nate,
  What gear oil were you running and how many miles are on it?

calvinhunt wrote on 03/23/13 at 19:43:17:



Glad I was able to help!!
This is what I use now, after seeing the error of my ways.

http://www.tractorsupply.com/en/store/travellerreg%3B-ford-tractor-all-mineral-9...
its cheap enough!!!!!

I knew about the yellow metal issue from working on tractors, but big dummy me didn't put two and two together on my jeep transmission until a couple months ago after reading about it on the 2a page.  

I used this rig that I bought at NAPA a couple days ago. I still ended up with a fair amount of fluid in places it shouldn't be but it was much neater than the funnel method I tried last time around. It's pretty quick too. I think I'll also be able to use this pump to drain the tranny, which should cut down on the usual mess.

Thanks gmcjr for posting the chart. I knew the 30 wt was on the low side of the 90-85 gear oil but since I have used it in my farm tractor for 11 years now working it very hard and no trouble I knew it would work in the Jeep.

My preference is a 40 or 50 wt engine oil but you could also use 100 or 150 ISO oil. Unless I have issues with shifting or something I will stay with the 30 wt non detergent engine oil. So far it shifts just fine now, we'll see how it goes in the summer.

Okay, I'm gonna poke the hornets nest here. First, I think the best oil for the T-90 is the type recommended in the service manual. In looking at the forums it seems to me that the "problem" transmissions are a minority of units with low mileage and that have been recently overhauled. Are the gold metal flakes caused by the oil or a mechanical problem? I agree that metal eating oil is not good for a transmission, but I think it will take many years before this will have any effect. Nate's example of the drum of gear lube is the perfect example. That's the lube almost every manual transmission and rear axle received from the 60s through the 80s. It was in every gas station and repair shop. Does it eat yellow metal? Probably, but how many transmissions actually failed because of it?

I reckon mines the other side of the spectrum, the transmission in my jeep has not been rebuilt since I've had it.(I'm gonna fathom a guess, by the way it shifts sometimes, that it hasn't been rebuilt as long as I've been on this earth. Btw, thats 30 years. The speedo currently shows 91,000 for what little that's worth.) I put gl5 in it a few years back, ran it probably 5,000 miles. I drained it a month or so ago when a similar thread was posted on the 2a page, the oil had a brassy tint.
I let some drain on my finger and then smeared it on a clean paper towel, there was brassy gleam (edited for better description) to it. It was enough to convince me. Your results may very.

That'll be interesting to see. I can't honestly say the brass I saw was just wear and tear, or result of chemical degrading. One the flip side, gl1 is cheaper. The Scottish in me likes that.