I know that some time ago we discussed at length the options along with the pros and cons of various gear oils for our transmissions.  There were even a couple of posts with viscosity index charts.  I remember that back then I came to grips with a decision to change to straight mineral oil, probably just automotive engine oil 30W, 40, or even 50W aviation engine oil).  Now I'm having doubts.  I know that we were afraid of the additives in the different GL-5, GL-4 (which claimed right on the container that it was formulated for gear boxes containing non-Ferris metals (yellow metals), and GL-1, etc. 
There were so many examples of different ones of us draining our gear boxes into a drain pan and seeing the beautiful brass-colored metalic oil, especially when viewed in direct sun light.  I have to wonder if much of the metalic source was not from the break-in of the newly overhauled transmissions.  Would we have seen the same metalic sheen in the straight mineral oil had we used that for break-in?  Has anyone done that? 
I'm undecided about what to do and need a little nudging.  Nudging welcome.

Calvin,
          After this thread in March, http://www.cj3apage.com/cgi-bin/yabb2/YaBB.pl?num=1363471481
     I replaced my unknown gear oil with the Nappa SAE 90 Mineral Gear Oil GL1.  The oil I removed was dirty but not brassy. Tranny is still working/shifting fine.
In the "Willys Overland Mechanics Manual" on page B1 it calls for (Trans. Gear Oil, Winter 80, Summer 90.) for use in transmission, transmission over drive, and transfer case.
On page B4 under the Transmission Overdrive section it states "Gear lubricants containing any extreme pressure ingredients, such as lead, sulphur, or chlorine compounds must not be used in overdrive units and over drive transmissions."
  Since the book calls for the same oil to be used in the tranny, T case, and OD units I assume that a lack of additives will not harm the tranny and T case. and the SAE 90 GL1 should be fine.
That's my 2 cents

The non detergent engine oil will be just fine. The GL-1 is about the same as 30W engine oil and mineral oil is the same as non detergent oil.

I have been using non detergent 30W engine oil in my Jeep since last fall and through the winter and summer and it shifts better than it ever has.

There was very little brass color to my old oil that was a GL-4-5 supposedly not harmful to brass but we have found that any oil with an EP additive is not safe.